It still sounds like a heat problem with the video card to me. You should get a benchmarking program that will stress the CPU and video card and if it crashes only during the video card portion then you're well on your way to a solution.

No beeps, no errors, no messages after a reboot. The only thing the logs say is "unexpected reboot". My mainboard does not have a IGP. Problem is i don't have other graphics cards or power supplies to test out which is causing the trouble. Which makes it strange that sometimes it does not happen after hours and sometimes after 2 minutes, like i said yesterday i played Tomb Raider two hours straight with no problems. So that confuses me most. 

People suggestion it's a heating problem, i am sure it's not :). Thats the first thing i made sure off. And a reboot after 2 minutes is for sure not a heat problem, processors don't heat up that fast when cooled.

Are there any programs to test the power supply?

Well try a piece of software such as future mark, furmark or any other software designed to push the GPU without it being a game. It sounds like its a 3d GPU problem but its unrelated to overheating. It could just be a fault on the graphics card which is only expressed when the card is used. I had the same issue, I could play 2 hours or 10 mins and there was no consistancy but your symptoms match mine and we both have Radeon 4xxx graphics cards.

http://downloads.guru3d.com/FurMark-v1.6.5-download-1965.html

Try that program, theres nothing out which stresses the GPU like it and if you can exclude the graphics card it really only leaves the main memory and PSU as potential problems. Its also the most likely explanation at present so it should be excluded first.

I found this via google.

"To make it so the computer doesn't auto-restart after you receive a BSOD (allowing time to write down the error code it gives), right click on "My Computer" and go to "Properties". In the "System Properties" window, click the "Advanced" tab. Under "Startup and Recovery" click the "Settings" button. Under "System failure" uncheck the "Automatically restart" checkbox. Now you will be able to see what your error code is with enough time to scribble it down."

Try that to see what the actual error code is.

Thanks for the tip. I tried furmark, did the stability test with extreme burning mode. Let it run for 10 minutes, don't know if thats long enough. But no problems of any sort. So guess my Radeon is ok.

option 1
try to scan your pc for viruses in safe mode

option 2
try to play with open chassis and clean your fans

I hope you will solve this problem

Something like that happened when I overclocked my CPU - it just restarted during gameplay. But this doesn't sound like that.

I'd check RAM and HDD if you are absolutely sure your CPU is not too hot.

It might have been said, but time the approximate length of gameplay until the crash/reboot. Do this a few times to get a ballpark estimate. Then try using the computer for basic stuff (internet, word processing, things that dont use the graphics card a lot). If you have the same crashes, it could be corrupt memory. If they computer is working fine, its related to the heat output of your video card, as it works harder when processing gaming graphics. Some cards have really crappy fans on them and have to be modified. What is the temperature of your computer while using Internet/Word, etc and what is the temp while gaming? Computers restart when they reach a danger point, to save the hardware. Even if you think you have your cooling worked out, it might not be sufficient enough, or perhaps a fan might be stopped by a cable resting on it.

Try to reinstall your gpu drivers.

Thanks for all the tips :). The power supply was the problem. Put in a new one and every thing is working like it should. A whole new world of PC-gaming opens to me now lol