Periodic freezes

This build has always froze on a periodic basis since I bought put it together. The first time a new video driver, a firmware update for the graphics card (which are no longer available), and a BIOS update solved the problem for a while. But it would still freeze every once a day maybe. Now I'm at the point where a game will freeze the machine on no longer than an hour. Doing things that demand little of the machine will still eventually freeze it in time. I saw some RAM on sale, so I bought it, but it didn't solve the problem. This problem has persist through two different motherboards, new ram, a PSU upgrade, an upgraded CPU cooler, several hard-drive wipes... I'm not sure how to diagnose the problem here. What happens is very gradual. First you'll hear the sound will scramble, or the peripherals will stop working. Then slowly applications will stop working one at a time, until the whole computer stops. no BSOD, just stops. Same with Memtest86+. There will be no errors, the test will just stop after about 10 minutes.

My current tech specs:

AMD FX-8120
Asrock 990FX Extreme3
16gb Kingston KHX1333C9D3B1K2/8G
Corsair TX650 V2
EVGA GTX560Ti

Any suggestions for testing or debugging much appreciated.