Light Problem

I have a program that loads a landscape with mesh & Octree.
I have set a red light(directional) ant that's the result:

http://www.ifrance.com/openGl/pics/ERROR1.jpg

But when i move the camera it becomes:
http://www.ifrance.com/openGl/pics/ERROR2.jpg

Does someone see the problem ?
Thanks

Chris

[Leyder Dylan]

Perhaps you've a problem with the normal ?