Question Machine reboots itself over and over

Oct 28, 2020
1
0
10
When I first start it up, it will make it to the login screen, then it shuts off and comes back on, only to be on for a few seconds and then off again, then on again for a few seconds, and goes into a loop. If I shut it down and 'let it cool', then try again, it gets back to the login and starts all over. I don't think it is a Windows thing, but a timing thing. I have disconnected the HD and tried to just stay in BIOS, but it has the same behavior; stays on for a few minutes the first time, then starts rebooting itself in a loop. I thought it was a heat problem but I've check the temps and they are fine. I tried different RAM. Same behavior. I removed the MB battery, hoping to flush out any stored problems. Same behavior. I believe it is down to MB, Processor, or Power supply. It is an all in one PC, with the external power supply like a laptop. How often do those go bad?

Thoughts?