Custom PC Restarts Occasionally

[C-Rex]

Over a month ago I built myself a new PC. and occasionally it restarts itself. This first happened about a week after building it - it is not a constant problem as I can usually run the PC for around two weeks before it restarts again, however, I would like someone to give me an idea of what could be causing it.

Here are my specs:

Operating System: Windows 7 Home Premium 64 Bit
Processor: Intel I5 3570k
Graphics Card: Geforce GTX 680
PSU: Corsair Gaming Series 2013 Edition GS 800W ATX/EPS
Motherboard: Asus P8Z77-V LX Motherboard
Hard Drive: Western Digital 1TB internal Hard Drive
Case: Cooler Master HAF XM USB 3.0 E ATX Case

Replacing any hardware is currently out of the question as I used student loan money to build the PC, and my family are currently very short on cash. I can't see it being a problem with the Power supply, as it provides more than enough power to the system, and I have only had it for around a month. I also don't see it as overheating, as I have ran tests and the highest temp I got was around 50 degrees.

 

[clutchc]

Normally, shutdowns are caused by a poor quality or too-small PSU. Neither of which should be your problem. Although even the best PSUs can have a lemon now and then. Do you have another PSU you can try out for awhile?
Be sure the PSU to MB connections are secure. Sometimes the 4/8 pin power connector or the 24 pin MB connector aren't in all the way and won't "snap" over the locking lip on the header. The result is a loose connection.
You didn't add an extra standoff behind the MB during assembly that you didn't use, did you? Sometimes that will cause a short if it is under a solder trace. Also check for anything loose lying on the MB like a missing screw or strand of wire.
Try the running system for awhile with the GTX 680 removed and the monitor connected to the integrated graphics. If it doesn't shut down on you after the normal period of time, it could be a defective gfx card.

 

[C-Rex]

Hello, clutchc. I haven't got another PSU I can try out, and I've made sure all of the connectors are in properly. I didn't install an extra standoff to the motherboard as far as I know - it already came with several non-removable ones which I screwed the motherboard into.

As far as I know, there are no metal objects lying on the motherboard, and there are no wires - most are either tucked away on the other side of the tower or tucked behind the graphics card. I haven't yet tried removing the Graphics card because I found it really difficult to screw it into the case.

Anyway, to eliminate any other causes I swapped the plug extension I was using, changed the PSU plug, because the tower backs into a wall and is often pushed against it, which could have damaged the plug, and I have also removed the external hard drive (whenever I had it plugged into my other PC, I would sometimes get Bluescreens).