I would recommend the following.
1. Clean the switches on the Lock Relay
2. Clean the Jones Plugs
3. Check your fuses

Intermittent problems can be difficult but persistence will pay off. First step is to hook one of these bulb testers to the Lock relay coil to find out if the problem is caused by Lock relay dropping out.

Bulb tester
https://pinside.com/pinball/forum/topic/reading-gottlieb-schematics#post-2634425
https://pinside.com/pinball/forum/topic/em-pinball-circuits-basics-to-not-so-basic/page/2#post-3184715

The Lock relays on some of the early 70s Williams machines were problematic for some reason. I had to continually fiddle with the 2 sw. that controlled the lites on my Fun-Fest. So, as noted above you should check the four sw. on yours.

Capture (resized).PNG

Hi Jesper
I agree with bonzo71 - also with @HowardR. Here I use more words. The working hypothesis by now is "The Lock-Relay gets activated - it should stay pulling until You toggle-off the pin / unplug the main power cord --- for some reason the Lock-Relay let go at a time --- see the JPG - "my switch-A" (on the Lock-Relay) closes as the Lock-Relay let go (((switch-A is closed when the relay is not pulling, switch-A is open when the relay is pulling))) --- the closing of switch-A lets current flow to the coil on Game-Over-Trip-Side --- the game ends" --- this working hypothesis may be right / true or it may be false / wrong. You must prove the theory right or wrong.

In a fully functioning pin happens: We play through all the balls - "my switch-B, wiper-blade" reaches the position - current can flow to the Game-Over-Trip-Coil.
In a fully funtioning pin it may happen: The player plays one or more balls (therefore my switch-C1 closes / is closed) - then he says "what a lousy game - I do not want to play the remaing balls - I start a new game --- he presses the Replay-Button, this makes the Coin-Relay to pull-in and my switch-C2 closes --- current can flow ...

When the Lock-Relay is activated it then must stay pulling forever --- the so-called "Self-Hold-Circuitry runs through "my switch-D1, my switch-D2". When this wiring is broken-off: The Lock-Relay let go.
Very, very theoretical: On the power-side-connection there is a broken-off wire somewhere - sometimes making contact --- sometimes not making contact - see my D4, D5, D6.

What we like You to do is "make Yourself an SteveFury -Test-Light - a car-bulb of 12 Volt put behind another car-bulb of 12 Volt - this sums up (12 + 12) to 24Volt - a consumer just like a coil.
Have long wires on the Test-Light - hook-on at Coil on Lock-Relay --- take the Test-Light through the open Coin-Door out into the open - lay the Test-Light on the apron (use a sheet of paper on the apron to have the Test-Light not into contact with metal). Then play, play, play --- when the strange fault happens: Question: Did the Test-Light quit shining ?

Remember "Working hypothesis" --- another hypothesis can be: Switch on steady-pulling Lock-Relay is no good, causing the strange fault to happen.
Well, we must start somewhere with testing.

See in the JPG - I drew a "Test-Light on Coil on Game-Over-Relay-Trip-Side" --- with this Test-Light we could learn "When the strange fault happens - does the Game-Over-Relay trip ?". Greetings Rolf

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Hi currieddog
I do write a post to come in MS-Word - I then login in pinside - copy my MS-Word-Text and paste in pinside. So sometimes I do not realize "in the time I was writing my text - a NEW post was posted". I also agree with You. Greetings Rolf

Not to worry, Rolf!