check the roms maybe , just like the pacman video john did. Might have been the gyruss.
the one where he compared the files to a trusted rom download source.
pulled this from arcadeotaku
Symptom: Reboots constantly
This is different than the bad CPU socket problem in that it wasn't stuck at RAM OK but rather as soon as the crosshatch then the text came on the screen it rebooted. The Reset line on CPU #2 and 3 came active for a VERY short period of time then the game reset.
Swapped out the CPU sockets. Subbed out the custom chips. Nothing. On a whim I turned off each DIP switch and found by playing with them that when DIP switch #3 at 6J was on it would cause the system to lock up when running or to constantly reset when booting.
Replaced the 74LS151 at 4K and the 74LS368 at 5A. Problem still there. Checked the EPROMs again. They were all checking good on the programmer. Swapped out the first 2 program EPROMs. Nothing. On a whim I burned a new set of Williams EPROMs (these were Namco) and swapped out all the EPROM.
Problem fixed.
plenty plenty more here - dip switches etc etc
http://wiki.arcadeotaku.com/w/Galaga_Repair_Logs