Hopefully finding out, that the light-sources or the suns have something to do with this bug may help fixing it.
Switching to OGL is a solution, too, but it breaks env-mapping (which is IMHO one of the major improvements of FSO). Let´s hope, that Taylor will succeed in implementing this to OGL soon. That would solve a lot D3D Problems.
EDIT: Sorry folks. Starfox described the sun-effect in another thread. So i think, my post wasn´t that helpful. And it seems, that the problem is much bigger and really hard to fix. This, or every coder already switched to OGL.