Galaga suffers from some horrible problems with sockets and resistor packs along with tarnished/corroded custom chip pins.
Replace the sockets (24/28/40 and 42 pin ones) with good quality dual wipe sockets then replace the flat style resistor packs with new ones.
Clean the custom chips, reinstall them, and you'll have a solid working foundation upon which to troubleshoot for any remaining issues.
RJ