[Eckhart]

i had a similar issue for quite a while. It started after some simple hangups and subsequent cold starts. This probably damaged the driver for my 8800GT Sli cards. I had to reinstall the driver after COMPLETELY cleaning out all what was left over from the previous one. I use Driver Cleaner.NET for that making several passes in a safe startup environment. This did it for me.
The other problem you have to look after is if this happens in an old campaign or a new one after reinstalling the driver. Sometimes the old campaign got damaged too during those restarts and have simply to delete it and start a new one.

Hope this helps,

Cheers,

Eckhart