On my old G5 2Ghz, I had the same problem. The solution (for me) -> plug a VGA monitor with DVI adapter to the graphic card, and it boot when I plug the cable... I know this is not a thing to do (to plug or de-plug to/from a graphic card when the mac is on, but that was the only thing to power on/down my mac) Without a monitor plug (or if a monitor is already plugged) , no way to start the G5 It's like there is no power, start switch does not go on. To shut it down, I needed to de-plug it after the "Shut Down clic" in the finder (elsewhere the G5 reboot but no shutdown) That was a really, really strange bug... Until the day I changeg my graphic card for a 6800 Ultra DLL and since that day the power button works perfectly. (same thing for the shut down, no need to de-plug video cable during the shut down process). Update: Few more informations, this G5 DP 2 ghz was a PowerMac with the "shut down" symptom. It works for few months without other problem and one day it shut down randomly. It can power on few...