PC keeps rebooting

Gateway PC 1GHz 256MB Ram with Windows ME installed. When it goes through a boot up, it almost gets to the Windows screen, then the screen turns black and reboots. It comes back and starts over again, this time going to a screen to where you can load it in SAFE mode. Sometimes it comes up in Safe mode, but most of teh time it just reboots. I have taken all the PCI cards out and have tried different memory, and get the same results. I even tried another power supply, thinking maybe it was power restting. Any Ideas?
Thanks……..