Freezes in games

I built my current PC about 2 months ago. It's been working flawlessly until 2 weeks ago when I got a complete freeze while Playing Killing Floor (after the game got a major update). A freeze which never ends if I wait and that doesn't let me ctrl+alt+delete or go back to Windows. All I can do is an hard reboot.

I thought the latest update to Killing Floor caused the freezes, but they happen in other games.

So far the freezes have been happening randomly in Killing Floor, sometimes in the main menu, sometimes in game. There is no pattern here, I can play for an hour without having a freeze and sometimes I get one after a few minutes of gameplay. I also got freezes in Call of Duty: Modern Warfare, Left 4 Dead and the Left 4 Dead 2 demo, but those are no as frequent as in Killing Floor. Another game, The Witcher, has not froze once in many hours of gaming. Running 3dmark06 doesn't cause freeze either.

My configuration:

Windows 7 Home Premium 64bit
Intel Core i7 2.66GHz
EVGA x58 SLi LE Motherboard
EVGA GTX 275
6gb of memory
Corsair 750watts PSU
2 Western Digital Hard drives (a 500gb with Windows on it, a 1tb with games/videos/music/photos on it)

I do not overclock.

My chipset drivers are up to date.
My BIOS is up to date.
My video drivers are up to date. (I also tried older versions and that didn't change a thing)
I have the latest version of DirectX
Windows 7 is up to date.
My audio drivers are up to date.

My temperature are all normal:

CPU is 32°C idle, 38°C in games.

GPU is 42°C idle, 65°C in games.

The freezes still happen even if I raise my GPU fan speed to 100%

I ran memtest and got no errors or problems with my memory. I tested the 3 memory sticks individually and got freezes anyway with each of them.

I still get freezes if I disable my anti-Virus (AVG) and Firewall.

I still get freezes if I disable my sound device (Realtek HD audio integrated to the motherboard).

The only thing I haven't tried is a clean Windows install, but I somehow doubt that will solve the problem.

I really don't know where the problem comes from. Software or hardware. Someone on another forum suggested it could be the Power Supply. Is there a way to verify that? Maybe the Graphics Card is faulty?