Game over lamp socket?

I agree with the ideas about game over or match. It is very easy to turn a socket when you replace bulbs. You need to carefully look at the sockets in the backbox. Another option would be to isolate each switch on the game over relay to narrow it down.

Buy a resettable 10a circuit breaker. Take the fuse out and put alligator clips from each side of the fuse holder to it.
Now you can have a mess about and try and see what the problem is without replacing fuses every few seconds.
Is there anything you have touched lately maybe a light socket?

Can you share a picture of the section on the schematic for the light circuit?

I looked at the schematic on my Lucky Hand—which is also a lightbox advance unit-based novelty/AAB hybrid like Neptune—because it works nearly the same. The sixth position of the ball count unit is what controls the game over light. If you never experience any blown fuses on any of the other ball count unit positions (1-5) the problem must be in the circuit for the game over light. Maybe there is a metal shaving or solder blob at the bottom of the socket that occasionally causes the shorting, hence the intermittent occurrence of the problem.