It is unlikely that this is a grounding issue. Ground problems on these games are notorious for crashing the computer, but unless you see the computer crashing (displays resetting or going dead or locking up), you're not getting hit with the ground problems from System 80 games. The computer just isn't involved with flippers, except for the enable relay.

I can't remember if flipper power on these games is daisy-chained or not. That is, the power lug on the flipper coil has multiple wires, and it goes from one flipper to the other. This would be the only thing they really have in common. If this is the case, there would be a loose wire somewhere. (Ground for the flippers is completely separate, since that comes in through the switches on the cabinet buttons.)