Hi Classicpinballs
random faults are non-pleasing - does it happen rare or seldom or sometimes or often or most of the times ?
When it happens rare or seldom or sometimes: I like to use a Test-Light - but I actually play --- the Test-Light is connected "parallel" to a "Relay in question" --- one side of the Test-Light is connected to "one side of the coil on the relay" - the other side of the Test-Light is connected to "the other side of the coil on the relay" - with this: EVERY TIME the relay gets (faulty) feeded by current - the Test-Light also lights-up. There are looooong wires on the Test-Light - I have it clipped-on at the coil - looooong wires - I can take it through the open coin-door out and lay it on the apron --- I have the playfield down and I PLAY - and have an eye on the Test-Light --- IF (if, if) the Test-Light ALWAYS lights-up when the fault happens: I am allowed to clue: The fault is in the feeding-line to the RELAY.
SteveFury made an Test-Light - see here https://pinside.com/pinball/forum/topic/reading-gottlieb-schematics#post-2634425 - two car bulbs of 12 Volts put one behind the other sums up to "Test-Light of 24 Volts".

Make Yourself such an Test-Light and hook it onto "Coil of P-Add-Player-Unit-Relay" - then play --- does the Test-Light ALWAYS lights-up when the fault happens ?

May want to try "quick and dirty test" --- the pin is started and some points are made --- bang with Your fist on the wood NEAR the P-Add-Player-Unit-Relay - does the fault shows up ? Greetings Rolf