I have the same problem as the guy above, same driver with a radeon hd 6850 on windows 7 64 bit and no mods. When I first start the game tells me I can run it on high, and I used to, but now I need to set it to low just so the ground isn't popping in and out of view.
Strange.. same card and i have no issues at all,
what processor and RAM do you use?
i5 2500k and some random DDR3 4x2 RAM
Could you both post your dxdiag please