Computer freezes up during gameplay

Hello everyone!

I've been having this problem on and off since i upgraded my computer with the new cpu/motherboard/memorys. The reason i said on and off is because it started when i had reassembled the computer, then all of a sudden it stopped out of nowhere, and a month or two ago it started again.

What happens is that when i play World of Warcraft (yeah, i'm THAT guy) the screen freezes up, do a quick graphical flicker or two, then i have to force reboot it, if i have music playing that also freezes sometimes, and sometimes it keeps going for a minute then stops. I've never had this problem when i'm only in windows or something, i even got the Bad Company 2 beta couple of days ago, and been playing that for hours straight, and that's a kinda heavy game for the comp to handle, and it only froze once then.

So this is getting sort of anoying, so now i turn to you good people for help, any assistance at all is much appreciated.

Oh, and tell me if you need any other information, as you might notice i'm new here. :)

Thanks in advance, Sebastian.

Could also be a heat or PSU issue.

Did the memtest and it didn't find anything, neither is there any heat issues and the PSU is functioning as normal.

All the drivers are up-to-date and the system is compatible.