I would start by checking the temps and voltages in the bios. How long since you cleaned the dust bunnies out of this rig with a can of compressed air?

 

It's been dusted up and down, I usually do that every month or so to avoid any fan blocking. I've reset the bios on the mb, but I usually can't hold the bios screen for very long before a shut off. Will try though, but could they have changed or reset randomly? or perhaps with a software update?

 

Get the temps and voltages first, it doesn't take very long, so you should have time before it shuts down.

It may not be conclusive, so you should also see if you can borrow a power supply to test with. Something in the 500 watt or higher range.

If it is heat, you are going to need some thermal paste, might keep that in mind too, if you don't have any handy.

 

Want to try something? Take the side of the case off and put a fan blowing directly on the CPU and fire it up and see if it stays on longer. That might tell us if it is temps, but not always.

 

of course, simple answer is always BIOS temps. But forced induction is usally a close second