When I start a game, initially it runs smoothly, but after a couple of minutes, the performance gradually decreases to the point of being unplayable (1-2 FPS). The sound also starts to lag at this point.

This does not happen every time I start my PC, usually exiting the game, rebooting, then starting the game again solves the problem, and I can play with perfect FPS for as long as I want. I could not find any deterministic reason when this happens and when doesn't. It happens in every game I tried (SWTOR, Diablo 3, Skyrim), and not even games, but simple applications like a browser or the Control Panel can get unusably slow. This is a brand new PC I bought three months ago, and this problem occurs since the first day I've been using it.

Could you provide any advice how to further diagnose the problem? I tried to reinstall Windows, and tried different video card drivers, but it did not help. It would be important to know whether this is a hardware or software problem, because I can use the warranty if it is a hardware issue. (I did not want to return the PC yet, because I can't reproduce the issue deterministically.)

Spec of the pc:

• Motherboard: ASROCK H61M-HVS
• CPU: INTEL Core i3-2120 3.30GHz 1155 BOX
• Memory: KINGMAX 4096MB DDR3 1333MHz KIT
• Video card: GIGABYTE GV-R685OC-1GD HD6850 1GB GDDR5 PCIE
• HDD: SEAGATE 500GB Barracuda 7200rpm 16MB SATA3 ST500DM002

I am using Windows 7 64 bit.

Thanks a lot in advance!