This is a Dell Optiplex GX620. When I hit the power button, it alternately turns on for a few seconds, then off for a few seconds, then on for a few seconds, and keeps going the same way, but never starts. I tried using a power supply that I know works. The capacitors on the motherboard are good, at least to appearance (none are swollen or cracked open or leaking) Have you ever run across this behavior before?