Good bet on the power supply. Check your 5V line RIGHT AT THE JAMMA CONNECTOR with a multimeter. It should be 4.95-5.05V. Most power supplies are adjustable using a little potentiometer. Try to adjust it within this range.
If that doesn't fix it, just replace the power supply. They're not overly expensive. Bob Roberts and Happ are probably the easiest places to get them. If you go with Happ, it's probably a good idea to contact Divermaster127 (he's a Happ distributor) on here as Happ's direct shipping prices are outrageous.