Computer turning on and off

[demonkoi]

I recently  rebuilt a pc it worked fine for a while but now when I start it, it turned on for a few seconds (the time on greatly varies) then it turned off and automatically turned back on (it takes varying amounts of time to turn on between 5 seconds to half a minout) it is stuck in a startup shutdown loop need assistance 

[ShepBook]

I'd start by re-seating the power cables to your motherboard. Maybe something is a bit loose?

[Petoovee]

If you let it sit for half an hour does it get further then?

 

Maybe something isn't getting cooled properly?

[AbstractAlbino]

Check your thermal paste if it's right and not too much or not enough. Also check your Cpu temps, and clean your pc from dust if you have any.

If you already done that, check your bios maybe you messed up something in power.