Pc shutting off randomly

Hey guys, I just recently put together my own desktop for the first time (with the help of an experienced friend), and so far, I'm really happy with it. The one problem I do have, is something that I actually noticed right away. While I'm playing a graphically intensive game (Guild Wars 2 on all high settings in this case) my computer can randomly shut down. The way it happens is really strange. The first time, it shut down within twenty minutes. The next day it didn't shut down at all. The day after that, it shut down after 2 hours or so. I thought it might be overheating so I downloaded speed fan, and monitored the temperature during gameplay. The Graphics card never went about 50 Celcius and the CPU was steady at 66 Celcius. When it shuts down there is no warning or anything. FPS doesn't drop, the computer does not slow down, it just shuts down with no warning or blue screen of death, etc.
Here are some stats of my computer.

Operating System
MS Windows 7 Ultimate 64-bit
CPU
AMD FX-8150 21 °C
Zambezi 32nm Technology
RAM
8.00 GB Dual-Channel DDR3 @ 802MHz (9-9-9-24)
Motherboard
ASUSTeK COMPUTER INC. CROSSHAIR V FORMULA-Z (AM3r2) 34 °C
Graphics
VA26LHDTV10T (1360x768@60Hz)
NVIDIA GeForce GTX 670
Hard Drives
466GB Western Digital WDC WD50 03ABYX-01WERA1 SATA Disk Device (SATA) 27 °C
Optical Drives
No optical disk drives detected
Audio
NVIDIA High Definition Audio

I have a 700 watt power supply as well.

The case itself has 4 fans initially, plus I bought a bigger heatsink and fan for the CPU to avoid overheating if anything. I really don't know the problem behind this.

Note, I also ran the Furmark program for 15 minutes, and my GPU rose and then sat at 70 Celcius and my CPU didnt even go above 50.

I'll be really grateful for all of your help!

 

What is the make/model of your power supply (PSU)?

It may be kicking out on overload, temperature, or could possibly have a manufacturing defect of some sort. The card requires a minimum of a 550 watt PSU, so you seem to have plenty of power. However don't rule out that you may be a bit short when factoring in the CPU, RAM, and HDD for wattage draw. Your CPU has the potential to draw 125 watts.

I have seen hard drive issues in the past which managed to shut down the PC. You could try installing the free version of HDSentinel, just to possibly rule out HDD condition, and temperature as a factor.



http://majorgeeks.com/Hard_Disk_Sentinel_d7520.html