Do you change graphic mode half way... ie start off in windowed mode then switch to full screen? If so this can bugger stuff up if you don't reload everything after a mode change.
Nope, right to full screen and loading the gfx from there on out.
I ran into something similar to this, which turned out to be a Z-buffer issue. Try changing your colour depth to 32? Try reducing your CameraRange?
Was it on a High end geforce?
I personally dont have a geforce6800 to test on (ti4200). I would like to get one, but they are all pciexpress, which would require a complete upgrade on my system. The thing is, the app works fine on my end, no issues with the gfx.
The issue is occuring on the designer's pc who happens to be a few states away from me. I have no way of testing this for myself and was hoping someone experienced something similar on a similar card.