I have a Radeon 32DDR and I have the same problem... or symptom. This simply may be an option not available to Radeon users. One person on a CRPG forum thinks Radeons are not compatable with NWN because his game keeps crashing. However, I run NWN on my Radeon and it looks good and runs fairly smoothly.
Maybe someone else has a more definitive answer on this. Since it runs the game well, I'll probably keep the card, however, I am wondering if I would see significant improvements with a GeForce 4 T-4200 over the Radeon. I'm guessing I would see improvements, both in performance and quality of image. I do get a few stutters here and there. My CPU should be fast enough so I'm guessing that the vid card is the weakest link at present in my system. Still, it runs pretty well in general.