The first thing I do with a galaxian board is swap the 2 proms @ 1H + 1K with eproms they can cause issues if 1 of them is bad. That or the 27LS00's on line 1 can cause odd issues with the invaders if they are socketed swap them around and see if the problem changes.
The galaxian boards have problems with the corroded prom legs and sockets also.
Also make sure, if someone else has worked on the board, they didn't install something backwards or wrong.