If you re-install don't forget to back-up the textures sky's models etc that you imported. After the re-installation you'll have to re-import them again. I had a problem like this way back when I was still using a 300MHz AMD K6II with 16MB EDO RAM (66MHz) and Bryce 2. I found no real solution to it. But my quess back then was that it was a memory issue, because it only happened on the more complex scenes.

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