It sounds like a video driver problem, but look at the cryptic system event log for further clues.
I have just replaced my motherboard, memory, CPU, HHD, ,GPU and now my pc shuts down as if there was a power loss. To get it back on I must switch off the power supply (on back of PSU) and unplug it for 15 seconds or so and then power up. I am then prompted to start windows normally of in safe mode. Then it starts and runs normally. This only seems to happen when I am running 2 monitors (one DVI and the other HDMI) and most frequently when online. Ran Benchmark 3D with out issue, temp got up to 68c.
PSU: Thermaltake TR2 600 W
MB: Gigabyte GA-990FXA-UD3
CPU: AMD FX6300 3.5GHz
RAM: Ballistix DDR3 1600 8Gb
HHD: Seagate 1TB 7200 RPM
GPU: Asus Radeon R7 250X 1Gb
Any help would be greatly appreciated.