System overheating?

Ok so i've had this problem for a little while now. My computer is about 3 years old or so and was custom built for gaming by me. I've had to replace the mobo a few times and bought a new one last winter. Everything was working fine but about three or four months ago my computer will randomly shut down, and i mean shut down like instantly. From on to power off as if someone pulled the plug. There are no warnings, BSOD or logs of this event. Just simply unexpected shutdown when i start back up as if you held the power button. It will come back up and start again right after it crashes. This tends to happen whenever i do anything graphic... at all. If i watch a youtube video (360p standard def) in fullscreen mode it shut down a few min into it. Forget playing games (although i was able to play starcraft 2 for about 15mins before it shut down.) So i have taken everything out cleaned it, reseated it, and took the plastic design cover off my gpu so the heatsink foils are exposed. Took both sides off my case and stuck a giant box fan on high next to it... no help. I haven't tested my psu but i doubt its the problem and i have my comp plugged into a serge protector and i'm not losing power or have any viruses. I used speed fan to set all my fans to 100% and used nTune and under clocked my entire system to the min possible values and volts and still same issue. When i tried to monitor my system temps read out like this:
GPU: 52C
Temp1:26C
Temp2:26C
Temp3: -128C (possible faulty sensor???)
Core0: 28C
Core1:26C
Core2:25C
Core3:25C
This is at normal web browsing and minimal usage (while i write this) and when under load the Temps and Cores stay at like 30-35C max and the GPU can get to 65 max. I watched and when the GPU was at 59C and climbing the system shut down. But thats a (if not warm) normal value for a gpu... so I don't get it. I've formatted completely and ran a memory check and chkdsk and they were all fine. I could run a system stability test through nTune but that won't tell me whats wrong I don't think anyway. So is overheating the reason the system shuts down? If so what should I do?
Specs:
mobo= XFX nForce 750i SLI thermal compound and heat sinks
RAM= x4 1gb corsair DDR2 with heat sinks
CPU= Intel Core 2 quad Q6600 2.4GHz with heatsink, thermal compound and fan
GPU= nVidia Geforce 8800 GTS SSC edition from EVGA with thermal compound, heat sink and fan
PSU= X3 Extreme 1000 Watt
HDD= seagate 250gb 7200rpm

Thanks for any help on this!

I also used CPUID hardware monitor and it gave me slightly higher temps for the cores and Temp values. However my gpu came back saying [TRIAL] so i'm not sure what that means...

Those temps are just fine.

I'ver never heard of your psu... and 1000Ws is way too much for your system. Hell i'm running a q6600 @3.2 and 1.4Vcore and SLI 260-216s on a 700W. You need barely 500W of quality pswu.

I'd suspect a psu failure. The gpu temp climbs due to load, load draws more power.... psu craps out.

If you can try something like furmark and/or primer95 you can put full loads on your cpu and gpu, and see if they crash you out.

You can test the psu with A multimeter, and use a spare 4 pin molex connector.

Driverheaven Guides: testing your psu with a multimeter

Prime95 32bit Prime95 (32bit) - 25.11 Download - EXTREME Overclocking
Furmark FurMark: VGA Stress Test, Graphics Card and GPU Stability Test, OpenGL Benchmark and GPU Temperature | oZone3D.Net

haha yeah I know now that its way to much but I was new to building comps three years ago and wanted to make sure I had enough... can to much power cause problems? here is a link to the psu ( Ultra X3 ULT40311 1000-Watt Power Supply - ATX, SATA-Ready, PCI-E Ready, Energy Efficient, Modular, Lifetime Warranty w/ Registration at TigerDirect.com ) I'll try furmark and see what happens. After that ill try the mulitmeter when i get some time (thanks for the link ) and let you know the results! Thanks

No, too much power can't hurt... Usually they hit max power between 20% to 90% load.

But you could be crapping out, as they do wear out.

ok so i ran furmark for 20mins fullscreen and no problems max temp was 78 C. I ran prime95 for two hours no problems max temp 47 C. But then i opened firefox and watched a youtube video in fullscreen mode and my computer shut down within 4 sec. Whats the difference? network traffic? No cuz i downloaded a huge file to test that yesterday and no probs. I think i'm going to run another memtest and maybe a hard drive test again. You may be right about to psu but i know its built to do this. When it detects a fault or temp or whatever its designed to stop ac power so thats why I think the psu is not bad something else is thats triggering it, although I can't be sure that its not just faulty and triggering for no reason. I'll try the multimeter test when I get home today if I have time. Thanks for your help so far!

Its not the psu now that you ran the 2 tests.... Unless its a quirk. Them 2 programs draw lots of power.

When it crashes, does it give an error?

Make sure your drivers are all upto date...

Use metest86+ to do them memory test.
www.memtest.org

Thats why this is such an interesting problem. There is no warning before it happens, no bsod or errors, and no log of an event except for "unexpected shutdown". All drivers have been updated as far as i know. When I get home i'll run the two next tests. I'll let you know after that

ok i ran a mem test, hard drive test, and another chkdsk and no problems there either. Could this perhaps be a virus that i'm not seeing? Do you think a format is in line here? Cuz im starting to think this could be a software or os problem. But i'd rather not go through the trouble if its not going to help. It still shuts down if i try to watch a fullscreen youtube video...

Not really sure....

Got another test for you... i forgot about it:
Folding@home - DownloadUtils

Its a memery tester for the gpu. See if it throws errors.

alright I ran both the cpu evaluation and the gpu2 evaluation. After completion of two proteins there were no problems and everything ran fine. Again opened a full screen youtube and dead within 5 sec of play. Also died at the end of a virus scan. But other than that everything is running fine whats going on?!?!?! I think i'm going to re-format this weekend to see if that helps. If not i'm going to RMA this board and check the psu for faults. Cuz i'm at a complete loss...

Yha... i'd go that route too.

Good luck

hahaha thanks i'll let you know after I do it and see if it helped...

Alright after the format everything seems to be fine. So i guess it was like a fail registry value or something lol. Anyway thanks for your help!

lol nevermind it just started to happen again so i'm going to RMA this board or possibly buy a new one and check the psu.... all that work for nothing!

but now it doesn't happen for youtube but if i try to launch a game like starcraft 2 in full screen it dies...

it also happens if the game is in a window...

RMA it.