Terrain Shimmer

Does anyone have any pointers on how to remove or reduce the terrain shimmer in the mid-distance? I've tried playing around with the terrain detail levels and mipmapping etc but can't seem to get rid of it. I know it can be done since it was practically non-existent on another system last year but I can't remember what sort of tweaking that had done to it.

The old system had a Voodoo3 2000. New system uses a GeForce 256 w/ 32MB and Detonator 6.31.