Do these artifacts move when the camera moves... can you get a camera angle that makes them disappear?
Reason I ask is they look exactly the same as some artifacts I have in my engine, but that is caused by a camera going under the terrain, and the camera render clip commands seem to be ignored. I suggest checking the camera render images, like 1,2,3 on the keyboard to paste the image. Look for the artifacts in the camera renders. Of course, that might be a bit difficult if you're using cube maps, maybe apply the cube map to a standard cube so you can see the pure render. I'd bet that it's caused by a camera rendering under the terrain, most likely the reflection camera.
Of course this doesn't help you fix the problem, I'm still trying to come up with a workaround for weird render issues. Personally, I'm thinking ray cast from camera to first intersection of terrain, and set the near camera range to that - so the camera kinda pushes forward through the terrain, but only with the render range. I think that will work, worth a shot at least if it gets rid of those artifacts.
