Does it happen on a particular ball? Does it always go to game over? Noticing some behavior like that might help narrow it down. For example, if it only goes to game over from the next to last ball, it could be something like the ball return switch is gapped too narrowly and is occasionally vibrating closed during play. If that were the case, on other balls, it would just quit playing and go to the next ball.

Being intermittent makes me wonder about a switch being gapped too narrowly, a switch tab being a little too close to a neighbor, etc. Does it happen more often if you pound on the cabinet or pf in a particular area?

Maybe the R relay hold-in circuit?
.................David Marston

If you have a partially/fully shorted coil somewhere, activating it might cause 24V voltage to drop so much that the hold relay drops out, triggering game over and removing power to shorted coil also. Check all coils and relays for signs of overheating (molten plastic or burnt wrapper).