Computer keeps rebooting

Hi,

So I have a little problem with my computer. It keeps rebooting when I'm playing some games. The problem started about a half year ago.There's no bluescreen or anything, just a hard reboot and then it boot it self up again. I tried to change the GFX, but that only made the problem worse, I had a Gigabyte Radeon HD 7870 2GB before. I've tried to remove the memory sticks, one at a time, but that didn't really help either.

The reason why I wrote "some games" above is that it works great while playing Civilization 5, but it might reboot after 3-4 minutes into the game while playing say Divinity: Original Sin. It's a bit hard to check, as it just hard reboots, but the GFX-card seems pretty hot when after the reboot.

I thought that it was just too much dust in the computer when the problem started so I have cleaned it out 3-4 times since and it has helped more or less, but the problem keeps coming back and the last time didn't really help at all.

This problem occurs both in Windows 7 and in Linux (Ubuntu 14.04). I've only tried it out with Civ5 in linux, but it has rebooted it self when i watched fullscreen video (the giantbomb stream, not sure if it's flash or not).

The dxdiag.

I'd be really happy if someone could help me with this.

PCPartPicker part list / Price breakdown by merchant

Type	Item	Price
CPU	Intel Core i7-2600K 3.4GHz Quad-Core Processor	$339.99 @ Mac Mall
Motherboard	ASUS P8Z68-V PRO/GEN3, Socket-1155	Manually added
Memory	Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory	$82.99 @ Newegg
Storage	Western Digital Caviar Black 500GB 3.5" 7200RPM Internal Hard Drive	$68.99 @ Best Buy
Video Card	MSI GeForce GTX 770 2GB TWIN FROZR Video Card	$328.98 @ SuperBiiz
Case	Fractal Design Define R4 (Black Pearl) ATX Mid Tower Case	$79.99 @ Newegg
PSU	Fractal Design Newton R2 800W PSU	Manually added
	Total
Prices include shipping, taxes, and discounts when available	$900.94