PC crash's when booting games and stress testing my GPU. GPU, motherboard or PSU problem?

[Joshgibbs]

CPU: i5 4690K
Motherboard: MSI Z97 G55 SLI
RAM: 8GB
GPU: Radeon 280x 3GB
PSU: Antec high current gamer 850W HCG
Windows 8.1
Overlocked? Nope
PC is 1 year old.

Whenever I launch games it shuts off instantly and restarts 2 seconds later, usually as soon as I hit the menu. No error message or anything. It also crash's just as I start stress testing with Valley Benchmark. I'm guessing it's either the GPU or PSU but I don't know how to find out which one. Temperatures are not a problem at all, I even cleaned the PC very thoroughly. No success.

I took the graphics card out and could actually play counterstrike on high settings without any crash's, I just used the motherboards graphics. This same situation has happened to me before, but it completely disappeared after 5 days or so but now it has returned. I haven't made any major changes to my PC, software or hardware as of recent, so... Anyone know what piece of hardware is the culprit? Thankyou all for reading.

I've also tried using all 4 different sockets on my PSU for my graphics card's cables and I've also tried clearing the CMOS. No success I even tried using a different PCIE slot. Rip.

 

[Demosthenest]

I would say psu as something similar happened to me not so long enough. Is there any chance you could try using another PSU from a friend? I cannot think of any other way of knowing which one is the problem.