Posting this whilst in safe mode as it runs without any crashes, i had this problem a few months ago, i simply switched extensions leads and which socket in the wall i plugged it into, and to my surprise it worked fine for months, until now.
I tried using a different GPU as i was told that might be the problem, but it didn't work. I also tried using a different hard drive, it worked fine for a few hours, crashed several times in a row and now its been working for 30mins or so.
Processor (CPU) AMD PHENOM II X4 970 (3.50GHz/8MB CACHE/AM3) - BLACK EDITION
Motherboard ASUS® M4A87TD/USB3: DUAL DDR3,SATA 6.0GB/s, USB 3.0
Memory (RAM) 4GB SAMSUNG DDR3 DUAL-DDR3 1333MHz (2 X 2GB)
1GB AMD RADEON™ HD 6870 PCI EXPRESS - DirectX® 11, Eyefinity 4 Capable
Power Supply CORSAIR 650W TX SERIES (TX650) 80+ ULTRA QUIET PSU