PC crashes when playing games

[BaiQiang]

Hi everyone,

I've been having problems with my PC. When I built it in November 2011 I had no problems at all and could run any game I wanted with the highest graphic settings. (Games like Skyrim, Battlefield 3,...).
After about 1.5 years, the computer started to crash occasionally after playing these games for a while. These crashes got more and more frequent, to the point that I can't play any games that require more recources than for example League of Legends or Dota 2. Lately even those games cause occasional crashes. When the computer crashes, the screen freezes and there is an very loud screeching noise in the headset. I assumed that it might be a temperature problem.
Therefore I exchanged the stock CPU fan with a Corsair H60 (120mm) cooler. (I also made sure to clean out the computer) Unfortunately, that didn't change anything.
I then returned my video card (ASUS GTX-570 DirectCU II (Dual GPU, 1.28GB)) and got a brand new Radeon R9 290X DirectCU II OC (4GB).
Despite these changes, the problem still appears.
I've also run the Prime95 Small FFTs Torture Test, while monitoring the CPU voltages and temperature. Even though I didn't observe any abnormal voltages or temperatures, I get a bluescreen after about 30-40 seconds of running the test.
I have no clue what could be the cause of the crashes and I'm hoping you guys can help me out. Thanks in advance!

My computer specs:

Windows 7 Professional 64bit
Intel Core i7 2600K BOX (LGA 1155, 3.40GHz, Unlocked)
ASUS P8Z68-V PRO (B3) (LGA 1155, Intel Z68, ATX)
Radeon R9 290X DirectCU II OC (4GB)
Seasonic X-660 (660W)
OCZ Agility (120GB, 2.5")
2 other normal hard disks
Corsair CMZ8GX3M2A1600C9 (2x, 4GB, DDR3-1600, DIMM)
Corsair H60 (120mm)

[the jolly roger]

might be overheating??? do you have the cpu stock cooler???