I suggest checking the lock relay. It immediately energizes when the machine is turned on and powers the game-over relay trip coil, which causes the machine to immediately go to game over when the machine is powered up. It sounds like that’s not happening.

When you turn the game on after being powered off, does the game over light come on in the backbox?

Three switches have to close at the same time for the game over trip (unlatch) coil to energize. They are on the lock relay (which is obviously closed the entire time the machine is on), the outhole relay, and the game over relay. Unless you’ve got a short somewhere else, either the switches on those relays are so close they sometime close the circuit when they shouldn’t, or the problem is further upstream, energizing or not energizing either or both relays when they shouldn’t be.

I would also check the wire connection tabs on the switches for the outhole and game over relays to make sure they aren’t so close that they can cause a short.

Another possibility could be the game over relay latch and trip mechanism itself. Does the relay latch and trip consistently when you work it manually?

I’m sorry, I think I was incorrect about the switch on the lock relay, I believe it is open once the machine powers on (being closed when turned off is enough for the game over trip coil to energize when the pin is turned on).