Windows 7 Random Shutdown/Restarts?

[sheepycat]

I built a new PC recently and it tends to shut down every 40-180 minutes and I have no idea why.
After checking the RAM, HDD and other things I was told it could be the PSU.
Is this a possible explanation, even if I'm sure it's a new power supply, and is there any way of testing?
If you need any more information I will respond as soon as possible.
Thanks.

 

[sheepycat]

I would start by monitoring your voltage and Temp's.

Do you know the easiest way to do this?

 

[sheepycat]

I just got a bluescreen saying: driver_irql_not_less_or_equal..
I think this is getting worse. Could it be the reason it's been restarting?