Computer turns off half way through boot

[Rodtaber]

My PC has began turning off during the boot process. It gets so far and shuts down. Then when you reboot it just about turns itself off before even getting started. If you wait for half an hour and turn back on it gets going but then shuts down again during boot. I have checked fans and are working ok. I tried getting into "safe mode" after waiting a while but it tried to go into safe mode but then it shut down again.
Help ??

[EEVIAC]

Are you using a video card or onboard video?  What is the make/model of the computer?

[Allan]

First guess is a bad power supply.

[killerb255]

First guess is a bad power supply.

Either that or a cheap/insufficient power supply.

Are you using a video card or onboard video?  What is the make/model of the computer?

...and this, especially if there's a video card (not onboard) involved, as the video card sucks up power from the power supply.  This goes double if the video card requires a four-pin molex or 6-8 pin PCI-e connection from the power supply.

[EEVIAC]

I'm thinking either the CPU or Video card (if any) is overheating...  She says after that waiting a half hour, it gets farther into the boot process than it does when immediately starting the computer back up....  My impression is that the cpu/gpu has had time to cool a little and that's why it's getting farther, after waiting..

PSU would be my next guess, however it's much easier to try another PSU than it is to add another layer of thermal grease to the cpu/gpu, so it would be better to try another PSU first, of equal or greater wattage..