What drivers you using Hajo? Generally, the 30.82 drivers from NVidia are a bit more stable than the 40.xx drivers. Some of the 40.xx drivers blow away textures randomly.
Also, you might try this; NOTE: this is not for everyone.
In the video7.cfg file is an option called FastVertexProcessing. Try flipping it from the default (it's either 0 or 1) to the other value and see if that helps.
You can edit the file with Notepad to make the change. If it makes it worse, then just re-edit and change the value back.