I'm hardly a tech guru or anything, but I had this problem;
1. When I alt tab, it has to re-draw the textures on the screen (this still happens with my current 2 gig ram machine).
2. It happened when I had too much spyware running in the background taking up space on the ram (again, on my old machine that had 1 gig only).
3. When I was running another program in the background that took alot up alot of ram (again, on my old machine that had less than 1 gig).
I'd try disabling the preload to the vid card, if one of the above isn't in question.