So looking at the service manual it seems to think IC5 or IC6 may be overheating if "Abnormality appears after playing the game a long time", so it says to heat one up with a hair dryer to see if it freezes. Guess that's what I'll do!
edit: but wouldn't those chips be hot? I mean, I can now set it all up and play till it freezes, and the cpu feels a little warm, but definitely not hot. IC5 doesn't even really feel noticeably warm.