This Gottlieb generation is notorious for having grounding issues, that often lead to inexplicable failures. Iirc the boards get their ground from the screws to the metal back. All machine parts run wires to the bottom of the cabinet where they are just pressed onto a big metal piece. Not soldered or crimped, just pressed. That point as well as the board GND connections often caused us problems due to vibration, corrosion and whatnot.
It's just a shot but maybe checking, cleaning and/or wiggling all these connections might help..