It only happens with some games; Shattered Horizon and Red Faction: Guerrilla both run perfectly, but Batman, Far Cry 2, and Mirror's Edge all have this problem.

http://www.youtube.com/watch?v=vL8d3rmWGCg

Any idea what's going on?

Recently I've tried overclocking my GPU, and I also unlocked the 4th core of my CPU. They both run at perfectly normal temperatures, so it's probably not the problem. I un-overclocked the GPU and disabled the unlocked core, but the problem is still there.

Some people think that I may have damaged the GPU when I overclocked it, but the temperature  was always under 50C. Although, I guess it is possible that it got damaged.

Specs:
Athlon II x3 435
Sapphire VaporX 5770
4gb DDR3 ram