Artifacts like these may be caused by the shadow settings or possibly the clipping plane.

If you're usng Dmap shadows try turning on the "Use Mid Dist Dmap" and bump the Dmap Bias to around .1

If it's caused by the clipping plane (usually if you have two surface close together), set the "Auto Render Clip Plane" to off and increase the "Near Clipping Plane" (1 is usually prettry good).