Computer crashes after a couple of hours browsing internet

[Nelinski]

Parts:
http://uk.pcpartpicker.com/user/Nelinski/saved/3aPy
My computer will crash after just browsing the internet for a couple of hours. (Not gaming/can't see it be overheating). Sometimes when it crashes the screen just goes blank like the pc is off (although the pc is still running).
The last few crashes it has said: "Please power down and connect the PCIe power cables". After a couple of restarts it will work correctly again for a while.
What is this likely to be/how can I troubleshoot this? I've got no other parts to interchange such as PSU/GFX card.
Thanks for any help on this.

 

[Tradesman1]

Sounds like either PSU or GPU headed south, might see if you have or could borrow a different PSU and try with that, if so then it's prob the PSU

 

[Nelinski]

I haven't got a spare PSU, is there a way I can check without swapping for another unit?

Here are some stats:

http://prntscr.com/2hkkop - This is when running battlefield on ultra as well as a few tabs open/radio playing. Side of case is open. Been running bf for about 15 mins, pc on for about 30 mins.