Power Supply Issue?

Hey everyone, I've been having some problems with my Computer, hoping someone here can help me out a bit...

TLDR: Computer is shutting off at seemingly random intervals. Used to be during graphic intensive applications, now sometimes just surfing the web. Any ideas as to what this could be? CPU heat is not an issue, and i don't think GPU heat is an issue. Any tests I could perform to see what is wrong?

Long version:
I built my own computer about 8 months ago. I had been wanting to do it for a really long time, blah blah blah. Here are the specs:

i7 with thermalright ultra 120 xtreme
evga x58 SLI
evga gtx 285
6gb ddr3 corsair dominator ram
corsair 1000hx psu
150gb wd raptor

Recently, the computer has been shutting off on its own. It started to occur only when I was playing graphic intenstive games, however, lately it has been occuring while I have been surfing the internet, or if the computer has been idle for an amount of time. It doesn't happen all the time, or on any sort of schedule. I had mild overclocks on the gpu and cpu, nothing to write home about. I thought maybe this was the culprit, however after returning the BIOS to factory settings, it was still occuring. I was wondering if anyone here had ever experienced this, or if anyone had any ideas what the problem was. I figure its the PSU, since the computer is shutting off when the GPU really isn't that active. Any advice?

Thanks in advance!