I'm experiencing with terrain... If i raise the terrain or even terrain that is already raised via height map editing will have itself clipped by the background land textures... See image below
http://a.imageshack.us/img802/9500/whatswrong.png

Any ideas why this is happening and how to fix it?

If I had to venture a guess I would say those are the old LODs, thought I haven't touched terrain editing so I'm not sure if it renders LODs in the GECK.

Actually I believe that is just normal clipping that occurs occasionally. Try adding an object to the render window (It can be anything) and it should stop. Then just delete the object.