Do see if you can beg/borrow a KNOWN working power supply, or a known working machine you can drop it into for testing. Power supply voltage issues can cause all sorts of oddball behaviors in an otherwise working MPU. Our own Solar Ride, for instance, was doing all sorts of bonus/drop target counting for switches not being physically hit, in fact I saw switches come up on the switch test that DON'T EVEN PHYSICALLY EXIST IN THE MATRIX, and eventually it just started to lock up until it would warm up. I finally bit the bullet and ordered a completely brand new power supply from Marco rather than attempt a rebuild (I already have one "carcass" from an attempted rebuild on a supply that was marginal, and didn't want to fiddle with doing it again), and it cleared the problem right up.

And yes, even while it's sitting idle, not in a game, the displays do flicker slightly when you press a switch. It's normal for any System 1 to do that.