usually clean with a razor blade
do you have a great magnifying glass ?
sometimes oxidation can be detected on a leg that might be inhibiting electron flow
there has to be a "video section" of the board that has a ROM nearby that's related
try following a trace(s) back from the finger edge connector towards a suspect ROM chip ?
make sure you lift that ROM out of the socket real even like or the leg(s) will bend
once the ROM is out, maybe a close inspection can reveal a piece of dirt or some visual problem with a socket
have you inspected the connector pins inside there
maybe one or more is weak and not gripping the game board tight enough
have 3 of the dual monitor boards with graphical problems that I cant fix.
haven't been able to find anyone who fixes them but its been a few years since I've looked
anyone on ebay advertising repair service on those ?
sorry that's all I got. hopefully someone here knows the answer and can chime in
please post a fix if you run across one or even if you find someone who can do repairs
one last idea. have you checked if the +5volts is correct on the power supply AND ON the game board ?
could be yours is low and that can cause screwy things to happen