PC freezes while playing

Hello everyone.

I bought a new system a few days ago and while playing games, my whole computer freezes and I need to force shutdown with pressing the power button for 5 seconds.. Sometimes i can run a game for 2 hours, sometims it crashes after 5 minutes. I tested my hardware again and I came to the conlusion, that the Intel HD 4000 causes the problems (there is no other graphics card installed at the moment). I read some other topics on this board but none of the solutions worked for me.

Here is my System:

Windows 7 64bit

ARock Z77 Pro 3

Intel i5 3570k

1TB WD Green HDD

16GB Corsair Vengeance DDR3-1600 RAM

LG optical drive

Corsair power supply with 500W

Games I tested that froze after sometime: Skyrim, Left 4 Dead 2, no other games were tested.

I tested around and those are my results so far:

-running games in windowed mode also freezes my computer

-updating all my drivers, including my bios didn't help, even with an old 8.xx graphic driver my game freezes

-i tested my ram, game freezes with a single stick of ram installed, doesn't matter wich one

-the system doesn't freeze during benchmarks with full cpu usage

-the temperature does not effect the result, games freeze under 35°C

-allocating more ram to the gpu over bios does not effect the result either

-software problems are probably not occuring, because the OS was installed a few days ago

I used a user defined screen resolution because my monitor is a bit bugged, but i ran the game in normal full hd resolution 1920x1080. Could this cause freezes? If this is not the problem i really have no more ideas why my computer freezes. If you need more information, please ask.

Thanks for reading and I apologize for my english.