Ugh, this game almost bankrupted me.
1. Galaga is super beyond sensitive to voltage. Try a switcher, see if it improves things.
2. Bum sockets, and pins corroding. If you have those, cleaning and replacing sockets.
3. lousy resistor packs, get those pups replaced.
I had a wierd repair which worked in rare instances in changing a time delay for a RAM write. If I can figure outthe root cause, I would release that one as a cure all for that condition.
but overall, I make the sign of the cross to ward off evil at Galagas.