try changing your sky sphere to draw back facing polygons.
i haven't been able to track down the exact cause of this bug, but it seems to appear/disappear from patch to patch and sometimes this helps it.
it seems to be when you have a plain that is textured with a transparent image that the 'edges' of it, when not backed by a some other form behind it (like your mountains), seem to allow the backdrop to show through.
sometimes this helps:
SET OBJECT CULL ObjectNumber, 0
cheers
-= i only do what my rice krispies tell me to do =-