Random Freezing (While Playing Games) Win7 64bit

Hi All,
I just joined Tom's, but I've used the site for a while. The reason I just now joined is because I'm having a problem, and yes, I have searched far and wide for the reason, but have never really gotten a good answer.
The problem only really occurs whilst playing games, but it doesn't really apply to all of them.
I got System Shock 2 to run on my system, and It never freezes. It seems to only happen on more modern games.
I read some article somewhere that said It could be UAC, so I disabled UAC and Defender (only because I have Avast running, and 2 anti viruses are doing me no good).
Like I said, it's basically a completely random full system freeze when playing modern games. No Ctrl-Alt-Del, no Alt-Tab, the only thing that fixes it is a system reboot.

My system is as follows:

Windows 7 64bit
Gigabyte GA-890GPA-UD3H
AMD Phenom II x4 965 Processor
Bios is stock
700WOCZ PSU
6 GB Ram (I have 1 stick of OCZ Platinum PC3 10666 7-7-7 @ 1.65v, and 2 sticks of Crucial 2 GB sticks overclocked to run at 7-7-7 with a couple notches into power)
2 ATi HIS 5750's Crossfire'd


Would appreciate any insight.