New built computer freezing, MSI P67

Hi guys, i just recently built a new sandy bridge computer. Everything in my computer is new except the graphics card. Sometimes my computer would freeze during windows mode, watching youtube or playing games. My computer doesn't freeze all the time, sometimes it can stay on for 5-6 hours without freezing. The kind of freeze im talking about is where your computer doesn't respond to anything, like my mouse doesn't move and all i can see is the display. Even during games it would freeze but not all the time. sometimes i can play for 2 hours straight without any freeze. But when it does freezes during games im able to use ctrl alt delete and it works but sometimes it doesn't work and i have to turn off my computer manually. I have not gotten any kind of blue screen. My temps are fine, i ran prime 95 and got around 48c for 4 hours running prime 95. What do you guys think it can be? Here are my specs and software im using.

i5 2500k
msi p67A-GD65
Corsair Veng 4gigs single stick ram
Corsair 850HX modular power supply
Samsung dvd burner
Samsung F3 1TB hard drive
Prolimatech Armageddon heatsink
ATI 4850 GPU

Software installed:

Windows 7 ultimate, Avast Anti virus, Steam,BC2, ATI 11.2 Catalyst driver, Adobe flash, Punkbuster.

Everything is up to date on software, including the latest msi bios.

As you can see i don't have that much software installed atm because i wanna see what was causing the freeze.
Anyone else having this problem with the same motherboard? or same problem in general.