Random PC Reboots

Hi

I would really appreciate any help with this problem I've been having since Dec.

My system:
i5 running at stock
Asus P7P55D
MSI TwinFrozr II GTX580 OC at stock
Corsair AX850 (was TX650W)
4x 2gig Geil RAM

Temps. CPU 28-31 at idle, SYS 30, GPU 46C at idle. My room is pretty cool.

I've had this system since 2009 and bought the GTX580 back in April. I used to run the system OCd i5 was running at 4GHz. I've been playing Battlefield3 for ages and have for long sessions and everything was ok.

Then at the start of Dec, my system rebooted randomly when I tried playing BF3. The TX650W eventually stopped powering up my GTX580 but somehow worked with my old GTX275. I bought a new PSU, AX850, and set everything up and the GTX580 is working again. However my PC reboots again when playing BF3, i get no more than 10 mins.

I ran OCCT on my cpu and its fine, but when i run the GPU test my pc reboots after a couple of minutes.

Now, surely it can't be the PSU its more than ample for this system and its new. So could it be a faulty graphics card or perhap a faulty motherboard or bad PCIe slot? I don't get any artefacts on either monitor (i'm running 2) and when my pc boots I only get one post beep which means everything is in order.

I've also ran GPU-Z and its reporting 12V = 12.17V, VDDC = 1.012V, VDDC Current = 19.0A and VDDC Power = 19.0W

I started loggin with GPU-Z while running OCCT GPU test and when my pc rebooted the GPU temp was only 57C!
Any help would great, i'm at a lost!

Thanks
Quyeno