Computer shuts down for no apparent reason

[Nisei]

Hi all.
I'm having some serious stability problems with my system.
At first I thought it was the OS but I went from Vista x64 to W7 x64 over a year ago now and the problem persisted.

My setup:

Gigabyte Odin Pro 800W
Asus P5N32-e SLI
Q6600 G0
4x 2GB OCZ DDR2 PC6400 RAM
GTX 560 Ti
4x Samsung Spinpoint 500GB

I've put all the clock speeds back to default so it's not overclocked at the moment. Changed memory timings to SPD settings so very relaxed.
Tried taking out and swapping RAM modules all to no avail. Besides that I ran Memtest+ for like 5 hours. No errors.
Went from an 8800 GTX to a GTX 560 Ti last week so the GPU can be ruled out as well.
I can run Furmark and Prime95 simultaneously and everything seems to be rock stable. Monitored with Core temp and everything's fine and stays below 50 degrees Celsius.
I can't think of a method to stress everything even further but as soon as I'm playing a game it randomly shuts down for no apparent reason. My last guess would be that the PSU is dodgy but why doesn't it shut down during the stress tests then?
Any help or suggestions are welcome (except telling me to buy a new MB and processor, I just can't afford any extra expenses at the moment).
Thanks in advance.

 

[Gigantopithecus]

Sounds like you should call these guys:

Seriously though, check the PSU. Anandtech reviewed the unit over three years ago so I assume it's about that old, if not older? And the review wasn't exactly glowing. You could also check the power coming from your wall. Is it good juice or dirty? It's possible your socket's providing dirty power, especially if you're in an older building.

 

[Nisei]

Thanks for the reply but how do you explain that the machine keeps running fine when I run Furmark and Prime95 at the same time? If there's anything stressing the PSU it would be running those 2 together.
I've been digging through the Asus forums (man, that takes ages! The forum is slow as a snail) and I've spotted similar reports. One guy dicovered it's a compatibility problem between the Asus and OCZ memory.
I'll be upgrading to the latest bios tonight and see if it's changing anything.

 

[Texashiker]

Random shutdowns are usually a power supply issue.

 

[betasub]

5 hours with Memtest isn't very long when you have 8GB to test. Have you tried just 2x2GB at a time?

Any particular game causing this? Gaming is different to stress-tests.

 

[Nisei]

First I'll upgrade my bios to the latest and report back. If it's still messed up then I'll let memtest run for 24 hours.
It's happening very randomly but there's a few things I can do to make it shut down for certain. One of them is playing a video with Windows movie player and jump back and forth on the timeline.

 

[finglobes]

The one time I had random shut down issues (every 15 minutes or so) it turned out to be because of my CyberPower UPS. I tore machine apart for two days, and the UPS was the last thing I could try and that was the culprit.