My Desktop Resolution is 1280*1024. If i Switch to 1600*1200 Fullscreen Mode and then press ESC, the whole Screen is corrupted. I can correct this by switching to fullscreen (e.g. 320*240

And maybe you could change your Rendering Engine to use LOD (Level of Detail) for those with slow Machines? (No, not for me, it runs fine on mine...)? Just an Idea... With this Technique you could also add more detail to the Scenes which can be Increased or decreased as one likes.