I got a good one.
Picked up a Flash Gordon and noticed displays were semi-ghosting. I took game to a pals place and he checked voltages which fluctuated a little on the high side. Unplugged all but one display and tested each 1 by 1 and nothing changed. Noticed the credit / ball count display worked great when it was lone one plugged in. Plugged displays back in and noticed only 6 digit high scores during attract mode. Turns out the displays were 6 digit instead of the 7 digit required. Plugged in a lone 7 digit display and all ghosting went away.