PC hangs randomly

Hi everyone, I'm having this trouble since yesterday, after my PC crashed and freezes, it corrupted my windows, and I had to reformat it, I thought it was a hard disk failure that caused this, I quickly back up my documents and I bought a new hard disk. After I re-installed windows, I thought the problem is solved, I go on with installing programs that I use, and surfing the web for some updates, then it freezes again, then after a hard reboot, I went back to installing stuff and it freezes again. So I did memtest, intel burn test, prime95, furmark for few hours and it didn't show any problems with it, I checked the temperatures and it's all below 50ºc under load, so temps mostly is not the cause. Then I went back to normal installing and surfing, it freezes again. This time I thought it was my SB burnt of something, then I went to my local store and grabbed a new motherboard and test with both hard disk, test including memtest, intel burn test, prime95, I tried to switch memory stick and it's still the same. I tried to switch all except the processor, but prime95 and intel burn test didn't show any errors on it. The freezes won't come out under intensive loads, but instead it will creep out at normal work like installing and surfing the web, that makes me confused.

I'm so headless and don't know what to do except going to my local store tomorrow and get a new processor. Any experts that had experience with this issue before? Please kindly help. Thanks in advance.

My specs,
AMD Phenom II X4 945@1.15625v I also tried putting back the voltage to normal but no luck, I've undervolt it for one and a half year.
Asus M4A785TD-M / Gigabyte 880GM-UD2H
2x2GB Kingston DDR3-1333 I've tried to put more voltage as some forum suggest but no luck.
On board graphics
Samsung 500GB / WD 1TB Blue
Seasonic 500w 80 Plus PSU

Thanks in advance.