I am having a very similar issue. In most cases, the system becomes unfrozen. Sometimes (such as last night when I was fighting a boss on WoW) it can freeze for 4 minutes before it returns the system back to my control. All apps will gray itself out and not be responsive. Mouse is still under my control. I have an nVidia 8800GT with the latest proprietary drivers (173). In my case, I have been thinking it is a SATA controller issue. But I will update when I get my new board and CPU.
