I'd check the stepping action of the player unit. If the "step" is too big you'll get a double step and get some pretty weird results. Manually actuate the plunger and watch how the bar grabs the teeth of the gear. There are adjustments/limiters to control how much travel each step can take. (use a sharpie to mark the original position of these adjusters should you mess thing up). I had a similar issue and found it was doing a double step.

When the game skips farther than it should, does the Player unit solenoid fire more times than when it works correctly?

If so, I'll need to see a scanned copy of the schematic. Sorry, not cell phone picures of it.

Hi mrbanjo
it is a strange fault that Your pin is doing. I do not know - I do guessing / loud thinking. The second player gets 1000 points - question: AAA: when the motor is running and the Player-Unit is stepped stepped stepped - or BBB: After the stepping has stopped - then out of the blue the 2nd player gets the points ?
Here http://www.ipdb.org/showpic.pl?id=2784&picno=15484 the right outlane gives 1000 points --- wild guessing: The switch below the playfield is adjusted tight --- the ball rolls over the trough switch below the apron --- shaking and the switch faulty closes --- the Player-Unit must be stepped from Player-1 to Player-2 to (not existing) Player-3 to (not existing) Player-4 to Player-1-next-Ball. At the time the playfield switch faulty closes: The stepping on the Player-Unit has been done - one step has been done so Player-2 gets the faulty 1000 points. This playfield switch is a wild guessing --- may want to sneak-in a stripe of paper to keep the switch permanent open - then test --- still the faulty 1000 points or no longer the faulty 1000 points ?
We may have to investigate more on the faulty 1000 points.

Skipping the second ball in a ONE-Player-Game --- not knowing so guessing: See the JPG (Your schematics) - the Player-Unit is stepped by actuating the "Add Player Unit Coil (encircled green)". When starting a new game: "enircled red" is doing the stepping. When stepping "in play": "Encircled green and encircled brown" is doing the stepping --- I believe ALL TIME LONG (stepping stepping stepping): The P-Add-Player-Unit-RELAY is steady pulling - it is made to pull-in (light blue) and stays pulling (dark blue). IF (if, if) Switch-motor-2B does NOT truely open when it should: P-Relay stays pulling AND another (round of) "stepping stepping stepping" on the Player-Unit faulty happens.
Maybe a friend can help - staring at the P-Relay murmur / whisper "pulling pulling pulling pulling" as long in time he sees the P-Relay pulling - and You look at the Score-Motor turning - I say: The P-Relay is allowed and must: Pulling while the motor does a turn of 120 degrees --- not shorter but also NOT longer - not for another 120 degrees.
No friend helping (?) --- make Yourself an SteveFury-Test-Light https://pinside.com/pinball/forum/topic/reading-gottlieb-schematics#post-2634425 - mount Your Test-Light like I show "JPG, light blue below the P-Relay-Coil" and have an eye on the light while looking at the motor turning.

The theory of "Switch-motor-2B does NOT truely open" - if the theory is true: You should have problems with coining-up --- as Switch-motor-2B also is involved in the coining-up. So to only check - only check the Switch-motor-2B: I added in the JPG "drawn pink / rosa in the left lower corner of the JPG. This "drawn pink / rosa" Test-Light would / will tell us about Switch-motor-2B truely opening or NOT truely opening. Greetings Rolf

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trough switch gaped too close...

Hi mrbanjo
my "not knowing - guessing - loud thinking" in post-8 is interesting (for me) I did some research and started an topic by myself. Post-8 is not helpful to Your problem - YOU mention at the end of post-12 the important fact "(seems like) the Ball-Return-Solenoid (((we often call this Outhole-Kicker))) fires BEFORE the bonus is finished counting down".

In a running pin FIRST the bonus does step down - it may need one turn of 120 degrees - it may need two turns of 120 degrees (- maybe even three turns when more than 10 bonus is possible). The motor comes to an halt (most likely so short in time that You do not see the halt) --- THEN the Outhole-Relay is made pulling - Outhole-Relay then kicks the ball over to the shooter alley.

See the JPG --- 1 --- 2, 3 (Motor is not yet running) --- 4 (ball rolls into the outhole and closes switch) --- 5 (You have bonus on the bonus-counter) --- the 6, Q-Relay pulls-in --- 7 (establishing Self-Hold-Current) --- lets say You have exactly three bonus --- bonus is counted down --- 8 opens (as motor comes near end of the turn) --- switch on bonus unit has changed from 5 to 11 as the bonus counter has reached Zero-Position --- motor runs out --- 8 closes again and 9 closes --- ball is still in the Outhole (so 10 is closed) --- 11,12 so 13 actuates and establishes Self-Hold-Current (through 14, 15).

WHERE is Your Bonus-Unit mounted ? Hanging on the playfield ? Look for the threebladed Make-and-Brake-Switch on the Bouns-Unit --- a rod mounted on the wheel does move the middle blade when stepping from One-Bonus down to Zero-Bonus --- all good with this switch ? Greetings Rolf

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Hi mrbanjo
Your (post-16) "Interesting clue tho ... The Outhole-Kicker is changing the Ball-Count ..." is an good observation. See the JPG - "my orange star near switch named 'P' " --- IF (if, if) this switch is faulty closed (all the time or sometimes): Faulty stepping on the Player-Unit (means faulty stepping on player and or ball). Where is Your P-Add-Player-Unit-RELAY mounted ? The "my orange star near switch named 'P' " Switch is the so-called "Self-Hold-Switch on P-Relay" - easy to be located: From one side of the COIL on P-Relay runs a short wire of any color (or maybe just a naked wire) - runs a short wire to "Switch mounted on the P-Relay" --- (have the pin toggled off / main power cord unplugged - Safety Reasons) - please inspect this switch - truely open when the relay is not actuated ? - truely closed when You manually actuate the relay ? --- NO DROP of solder fallen at the solder-points (making faulty connection) ? - NO DOGHAIR crap of wire fallen there () ? - not a short stud bent on the switch making faulty connection ? Inspect this switch --- then test again.

Your (post-17) "ball count trough switch" - what is this ? May I assume "Trough Switch that closes when the ball is kicked from the Outhole to roll over to the Shooter-Alley" ? Please write about.
IF (if, if) "my orange star near switch named 'P' " switch is faulty closed when the P-Relay is not pulling" - at the time the pin does make the O-Relay to pull-in - see the JPG: Upwards in the JPG through "my orange star near switch named 'P' faulty closed" - upwards and to the left - the P-Relay ALSO, faulty pulls-in (((and the P-Relay then makes the Player-Unit to step - faulty at this time))).

(((I put more colored marks into the JPG --- maybe we must do some more research - for the moment I hope for luck --- inspecting and cleaning and adjusting "my orange star near switch named 'P' " switch makes the fault to disapear / vanish))) Greetings Rolf

P.S.: In the JPG - "my blue star - three-bladed Make-and-Brake-Switch on the Bonus-Unit": Shown in the JPG is situation "NO Bonus on the Unit" --- does this switch truely work ? (No drop of solder - no doghair crap of wire - no bent stud ? --- Do manually step up and step down and watch the action.

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Hi mrbanjo
thanks for "clarification" and looking-up the "stuff in question". Yes, little gremlins are at work - are we allowed to say "P-Relay faulty pulls-in and faulty makes the Add-Player-Unit to step-up" ? Do You have proof ? Did You / do You see the P-Relay pull-in ? Or did You / do You have the playfield down but have mounted an Test-Light (mounted parallel to Coil on P-Relay) lighting-up ? If You do not have proof we are not allowed to say "P-Relay faulty pulls-in an so the Unit is stepped". There is ANOTHER way to step the Player-Unit - when we start a new game: The stepping of the Player-Unit is (starting a new game) done WITHOUT the P-Relay pulling. See the upper right corner in the JPG: I wrote an "burgundy red 'P' " to the switch named P in the JPG --- when playing, changing player / ball: Through "switch named P" upwards flows current --- but when we start a new game: Upper right corner leftbound through Switch on S-Relay and/or Switch P5B - then Switch SB2 and leftbound - current flows to make the Player-Unhit to step.
IF (if, if) "Switch named SB2" is faulty and the little gremlins make the S-Relay to pull-in (the little gremlins do not make the P-Relay to pull-in): Faulty stepping on the Player-Unit.

I wait for You to write "have proof" that the P-Relay faulty pulls-in (((then we look at the wiring to the Coil on P-Relay - "Through Switch" or "Self-Hold-Switch on P-Relay" --- these two wirings I see in the schematics - we then start to unsolder wires (one by one) until the fault no longer shows up - respectively sneaking in a stripe of paper in both switches to keep them open))).

As I am waiting for You to write "have proof": Lets do some theory. See the lower part in the JPG. Lets say You have THREE bonus on the bonus unit - the motor is not running - You loose the ball - the ball rolls into the Outhole and so closes the "Ball Return Switch". Along "my green lines" runs Initial-Current and the Q-Relay pulls-in - a switch on Q-Relay closes to make the motor to run. The motor starts running and after turning maybe 1 or 2 degrees: "Switch motor-1C" closes and keeps the motor "to run for the rest of the turn of 120 degrees - actually to 118 / 119 degrees". ALSO "another Switch at position motor-1C (encircled rosa / pink)" opens and cuts "my green wiring" - no problem as the pulling Q-Relay keeps closed its "Self-Hold-Switch" - along "my red lines" Self-Hold-Current to the Q-Relay is flowing. The motor turns further - the motor is made to run by TWO switches closed: "Switch closed on pulling-Q-Relay" and "Switch-motor-1-C (Self-Hold-Switch) on the motor". The motor turns further - comes towards the end of the turn --- Motor-Switch-2B opens and so cuts "my red lines, Self-Hold-Current of Q-Relay" - Q-Relay let go - opens its Self-Hold-Switch in the red lines - also opens its switch that was keeping the motor to run --- no problem for the motor: As motor-1C still is closed: The motor can turn further --- finally coming to 118, 119, 120 degrees - motor-1C opens and motor stops.
While the motor was turning and the Q-Relay was pulling: Not shown in the JPG: The Bonus-Unit has been stepped down - when stepping from ONE-bonus down to ZERO-Bonus: In my green lines the three-bladed switch moves /has moved. Then another feature can start: Feature "Ball is in the Outhole - no bonus - the pin actuates the O-Relay to do some stuff" - the rest of the feature is alike "feature Q-Relay" as far as motor turning and establishing Self-Hold-Current. Do You agree on the funtioning of such features ?

Back to "troubleshooting": Please always make some points (tens / hundreds) - do NOT make bonus --- the ball entering the Outhole therefore directly starts the O-Relay-feature.
Do I remember correct (?) - did You write "faulty behaviour starts BEFORE bonus is completely stepped down ? Lets simplify for a while - lets do testing "NO BONUS on the Bonus-Unit - the O-Relay must pull-in when the ball enters the Outhole - whats happening ? Greetings Rolf

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Hi mrbanjo
great - You have found the fault. I would have to make some crummy assumptions - the metal housing below the Outhole has some more connections - many "ifs" and "maybes" --- lets say "we live with the miracle - no theory".

I do know on Williams pins "metal housing of a stepper unit can be part of a circuitry / wiring" so the following this is loosely "same logic / stuff as Your Outhole-housing". To start the explanation --- first: There is a big difference "AAA Gottlieb" - "BBB Williams and Bally" in the logic of wiring - making the circuitrys.
AAA: From transformer-power-side runs a wire to fuse - then wire to a switch - another wire - maybe another switch - maybe another wire - maybe another switch - to Coil on a relay / on a stepper. The other side of the coil(s) have an direct connection to "transformer-return-side-common-side".
BBB is wired opposite: Transformer-power-side to fuse to "one side of coil(s)". Switches to actuate the coils are mounted in the returning-to-transformer-common wiring (on other side of coil connection to transformer-common).
Williams sometimes has the last wire (then connecting to transformer-common) - has screwed-on the wire onto the housing of a stepper --- all metal parts used as housing / used to mechanically step the unit --- all these metal parts are hooked-on at transformer-lug-common - part of the wiring. So far I know of Gottlieb-Bronco ALSO using the housing of a unit - all metal parts - using as part of the wiring:

wizardblom here https://pinside.com/pinball/forum/topic/bronco-coin-unit#post-4167859 started the topic and asked about an loose wire. On the bottom of the picture in his post-11 we can see the mentioned loose wire. In my post-9, second picture I showed "Williams solution, Shangri La: Wire attached to the metal housing" - I suggested to look for an screw that should have an wire (the wire) attached.
THEN - edrandall , post-12 showed a picture of the unit in his Bronco pin --- and we can see a screw on the metal stuff - housing of the unit with an wire (the wire) attached ...

Williams and Gottlieb Bronco use metal parts of the housing WANTED / NEEDED for the pin to function - the situation in Your pin was unwanted - is an miracle.

I like to use "Test-Light(s) mounted on a coil on a relay like shown in the JPG in post-23" --- especially when there is an "seldom / rare" fault to detect. I don't like to have the playfield lifted and for hours I chase the fault --- so on "relays in question" I hook-on Test-Light(s) - long wires coming out of the open coin-door - Test-Light(s) placed on the apron - playfield down and I play (having an eye on the Test-Light(s) ) --- and when the rare fault happens: Was there a difference in the pattern on lighting-up /Test-Light(s) ? - so I can play and narrow down on the problem. Greetings Rolf

Very interesting as I am having EXACT SAME issue with my 73 Big Shot. (see this link for video):
https://www.facebook.com/groups/347744179059319/permalink/487725468394522/