Well i finally jumped over to pc-gaming. I got rid of my Xbox360, because 99% of the games are on pc anyway, and kept my PS3 for the exclusive and Blu-ray. So i build myself a nice game-system. A Core2Duo E8400,  mainboard Asus P5Q SE Plus, with 4GB of RAM and a Sapphire Radeon 4890 1GB, using an 360-controller to play the games. All powered by a Thermaltake Thoughpower 600W powersupply. Everything runs silky smooth with no problems, and also games run perfectly in the highest resolutie supported by my monitor (1080P).

But i have one problem, while playing games my systeem just reboots itself. After that it gives no error messages what so ever and starts normally. I have no idea what the problem could be. I am running both XP and Vista (dualboot) on my system and the problem happens in either one of them. Strange thing is that it does not happen always. Yesterday i played Tomb Raider Underworld for 2 hours with no problems, while the day before it made my system reboot 3 times in half hour. After that i tried Prince of Persia and after 10 minutes BOOM reboot. Same happened when trying Fallout 3, playing about 15 minutes and again a reboot.

So i am puzzled. All drivers are up to date, alle service packs are installed. When i play games all other applications are closed. I know my system is not getting to hot. The 4890 did not get hotter then 75C (167F) and my E8400 stayed nicely below 45C (113F). Also both Windows versions are freshly installed, so there are no virusses or other crap on it.

If anybody got ideas what it could be, i would be gratefull for ur help :)

you might have picked up a dodgy RAM stick - try run a diagnostic on it

sounds like there may be a memory leak of some sort, ans since it happens with both version of windows we can rule out software issues, it may be perhaps related to hardware.

Maybe you could remove all ram modules, put one at a time, and each time run a game that crashes oftenly to test, of course you'll have to put the settings down, that way you can at least pin point which ram module may be faulty.

I recommend memtest (http://www.memtest.org/ ). It can pinpoint RAM problems, but also picks up on MB problems too.

Alternatively, the power source could be faulty, not handling the mild (or not) power spikes that could happen in the grid. Is someone welding a lot in your neighbourhood?

Sounds like my pc sometimes. It's likely a overheating issue.

Maybe you need to tighten your cpu fan ?

Is the video card overclocked and are you testing its temperature in-game or just on the desktop?

Last time that happened to me, my PC was overheating. Every time I booted up a game, it would shot down shortly after because my GPUs started to warm up and overheat it.

As people have said you basically should
a) monitor the temperature, which I understand you've done
b) perform a deep memory test
c) check the stability of power supply. Do you have a spare power supply unit lying around, to make a quick swap test?

Thanks for the suggestions. Temperature is for sure not the problem. I made sure my system is cooled enough. Nothing is overclocked so that can not be an issue either. The memory test seemed fine. I think my power supply has problems. When playing Fallout 3 my computer just shut down after a few minutes and i could not turn it back on for a few minutes .

Do you get any beeps on bootup? I had a recent problem with my GPU and it would reboot in 3d games. Try whipping out the GPU and then testing to see if the issue goes away when running 3d software with the IGP.