This is almost guaranteed to be a graphics hardware or driver problem. My ATI machines used to do it all the time. My updated NVidia 8800 series cards don't do this except on one machine, and that one only if I try to load 4+ WoWs. Basically, what appears to be happening is that the z-buffering is suffering some calculation problems and possibly some of the multi-pass texture engine is having the same trouble. You can probably drastically cut down on the artifacting by disabling "Specular Lighting," "Full-Screen Glow Effect," "Projected Textures" and setting the sliders to "low" for stuff like Terrain blending, Shadow quality, Texture filtering and Texture Resolution.

The big two are Projected Textures and Specular Lighting as those add a lot of extra passes to the rendering.

See if that helps.