White" Terrain Issue

I've been playing with TSE for a couple of days (so I have the latest HEAD). The legacy terrain is giving me some problems. The entire terrain turns white under a couple of conditions, and when this happens, the performance drops to about 6 fps. This white terrain occurs when a normal amount of terrain is visible, e.g. sky visibility = 500. If the visibility is reduced to about 100 or less, everything works fine... terrain textures are visible, lighting is normal, and framerate is above 100 fps. Moving around the terrain at a slow speed does not cause any problems. If, however, I fly the camera around the stage at a relatively quick pace, things will work fine for a few seconds and then the terrain suddenly turns white and the framerate drops. Relighting the world returns the terrain and framerate back to normal.

The terrain appears to be white due to terrain textures being additively blended, I think. In the image below, the right side has been darkened to show that not all of the terrain pixels have been clamped.


It seems like it might be related to a hardware bandwidth issue, since the more terrain visible or the quicker it is becoming visible seems to cause the problem, but I don't know why that would suddenly make the terrain texture blending change and kill the framerate. It seems like I should be able to fly around an empty terrain even with sub-par hardware. :) I haven't messed with Atlas to see if it behaves the same. Anybody else run into this problem?

Computer specs:
1.50 GHz
512 MB RAM
GeForce FX go5200