Steam Causing My PC to Shut Down?

[Inkwright]

Lately I've been experiencing random shut downs while playing Steam games. I can play for thirty minutes at most.

My rig is as follows:
AMD Phenom II x4 910 - 2.6Ghz (Quad Core)
8 gigs of Ram
GeForce GTX 770
Windows 7 Ultimate - 64bit

And I know it isn't overheating because I've run all the core temps and GPU temps and absolutely everything runs cool. I have a large gamer tower with 2 x 120mm fan, 2 x 140 mm fans, and 2 x 230mm fans - so the air flow is absolutely fine. My CPU's never get over 33-35C.

It also doesn't seem to be a power supply issue.

I can play games through Origin or other independent games for hours on end with absolutely no issue. But as soon as I load up a Steam game, the PC just shuts off after 30 minutes of play. I have no warning, no indication - temps remain normal and never spike, always running cool. The PC just goes off. The only way to get it back on is to flip the power switch on the back of the computer (or sometimes unplug the power cord) and then turn it back on.

Games that I know trigger the problem:
Lego LotR
Skyrim
Bioshock Infinite

Games I play through Origin with no issue:
Battlefield 3 (now Battlefield 4)
Dead Space 3
Dragon Age
The Titanfall Beta

I can also play Elder Scrolls Online and Diablo 3 without crashes.

Basically, every game I have played independent of Steam has worked just fine. It's only Steam games so far that cause the shut down. It's incredibly frustrating because I have a lot of games I want to play on Steam but can't.

I can't pass a single level of Lego LotR now because it crashes and I have to start over.

If anyone has any advice or suggestions, please respond. But if you're going to suggest it's overheating or a PSU problem or something wrong with my Ram, don't waste your time or mine. Those things have all been tested and are operating perfectly normally.

Thanks!