please help me out this problem?

[DeFender]

I got a core i5 3570 3.4Ghz, 4GB Ram and a Galaxy low power 9800gt 512mb (a second-hand good i bought 2 months ago), the power supply is enough for all components and the OS is windows 7 64bit. I recently encounter freezing system when I play game randomly (everything just frozen but the sound is still ok) and i have to hard reset the pc. I have checked thoroughly all hardware with Burn in Test software and found no problems. I also up to date my graphic driver and lower cpu speed but I still issue the problem. Do you have any idea what kind of thing is that and what is the best solution to rule out this permanently? Thank you in advance.