Got the same Problem without Alt-tabbing. The Game freezes sometimes for 5 seconds then turns black and then im back in the Game with the Clouds looking as in the first Post mentioned. Not enough GPU Ram?
Specs:
Q6600 2,4 Ghz
ATI 4850 GPU with 512mb Ram
4GB Ram
Windows 7 64bit
Game settings: Textures High, Visibility medium, Scenes medium,
Water low, Traffic off, HDR on, AA off, AF 4x