Check the memory in the system. If you have had one go bad on you it can make all kinds of havoc for a Mac. They are very picky about ram working or not.
Also, look online for a reference to the "secret" OpenFirmware controls for a Mac. If you hold some odd key combinations (I think its like command-O-F) you will get into the Bios control. But be warned, you can completely screw the Mac if you change any of the wrong settings. I myself have not dabbled in it, just read about it a bit. I don't want to kill my G5 for the fun of messing around.
I would also disconnect any extra USB or Firewire devices too. Until there was a recent firmware update, my G5 1.6 would have some issues forgetting the USB was there and the mouse and keyboard would stop responding completely. Turned out that a simple fix was to use a powered USB hub, or to remove the original 256megs of Apple ram that was in the machine and just have my 2 x 512 sticks and it would behave itself. No big deal though now, a couple weeks later, Apple fixed the firmware and its been peachy ever since.
Good luck to you.
