Something is telling your power supply to "reboot". The easiest way to determine what is causing this is to unplug things one at a time.
Turn your PC off and unplug it.
Then remove the power connections to everything except the motherboard. It should run through the POST examining everything on the motherboard and the video. You should see your BIOS splash screen.
If it continues to reboot, it is possible there is an erroneous setting in your BIOS.
If not, plug things in one at a time and reboot the PC. When it starts again, you have your "guilty" device.