Have you tried turning 'number match' off to see if that makes any difference ?
Is the game over relay coil burned ?? (maybe too weak due to age/heat to do its job properly ?
It sort of sounds like this coil could be too weak ?
I notice on the schematic for my '65 Gottlieb Thor Bred there is a "Last Ball' relay , but don't understand the purpose of it as yet . Could that be a problem with your game?
I'll post if I think of anything else and good luck .
