Some good news: After spending about $1500 for new test systems we finally found a system where the problem could be reproduced.

We believe that it's a bug in the nVidia Win2K drivers. It does not happen with any other nVidia drivers, such as the Vista drivers, even on the same system. The Win2K drivers are used for XP.

Hopefully with this new information we can find either a workaround or a way to get it fixed by nVidia.