Random Freezes / System Lock Ups When Playing Games Or Watching Videos?

The problem is that I will get a system lock up, or freeze when playing games like Portal 2 or FM2012. The freezes are very random however they occur the most when trying to play Co-op in Portal 2 (say about 80% of the time when trying to play it). However with the likes of FM2012 or other games, I can literally play for hours on end without a crash, but other times a crash could happen within about 5 mins.I also seem to randomly crash if I watch a HD video in full screen (1920x1080) however I cannot recall such a problem watching it minimized (using VLC). I also used to get random crashes while browsing the internet on certain websites, however I haven't had this issue since upgrading my RAM.

When the PC does crash, the monitor will just black out and if sound was playing at the time it will usually loop or repeat. The only thing I can do is a hard reboot and upon restarting the PC it will say about the unexpected shut down and to start windows normally however I don't seem to get any minidumps. I've had this problem now for quite awhile, and at first it was something I just overlooked as a random occurance but now its starting to drive me crazy! My system specs are as follows:[CODE]