Prime95 - PC Crash! GPU? PSU? RAM?

Hello Folks,

6 months ago, i bought/built my own rig (mid-high end), here are the specs:

ASRock Z68 Pro3 Gen3
Intel i5 2310 2900 Mhz
Lucid AMD Radeon HD 6900 Series
4 x 2Gb Corsair XMS3 DDR3 1333 Mhz
Xigmatek 500W 80+

Here's my problem: I'm suffering several crashes/freezes/lock-ups and tried eveything, with no results.

2 days ago, I decided to try some torture/stress test softwares, Prime95 | Memtest86+ | 3DMark etc.

The first Memtest showed only 1 error, which was enough to be confident that there is a RAM problem somewhere.

Then I tried Prime95 with the "Lot of RAM" test, results: with 4 sticks, it crashes! with 3 sticks, it crashes! with 2 sticks, it crashes! with one stick left, I thought that the 3rd could be the problem!

I then returned to Memtest to test RAM sticks one by one, the first one was error-free, in the seconde RAM I bumped on a error, which was one enought for me to test it in Prime95 to be utterly sure! But SURPRISE! The test is going well for it  

I hope i've been clear, if there's any clarification you need to help solve that problem, i'd be more that glad to tell you!

Waiting for you positive answer, THANKS!