first thing I would check is if board is getting proper +5 volts and a quick scan to see all the grounds are intact and clean or
check the +5 volts on the output of the switcher and then again ON THE game board for accuracy
back of finger on each rom after game has been turned on to detect if ROM is getting hot
if one is heating up, could be problem with the ROM or the itty legs there
had a Jr pac once that had similar video error and a good ol' heat sink on there wicked away the heat enough to let the video display correctly
ran fine after that for long time and then finally sold it
if that was my board, could figure its already broke so maybe I can bring it back
some say to reseat the ROM's and I would try and detect any dirt or oxidation inhibiting electron flow through the ROM seats and legs
dremel tool can help clean the legs but hard to not let the spinning brush get caught in the legs and throw the rom out of your fingers
carefully lift off a ROM and inspect the legs for oxidation/anything obvious
who knows, maybe your +5 v is low and just needs to be turned up
there are guys on here that might be able to fix is for you
I just never bought an oscilloscope and managed to fumble through what monitor chassis I did get fixed
good luck