Roger, but is getting occasional stutters *without* the frames dropping badly normal?
Yes. That simply means the video card can be running out of resources.
Sounds like you have triple buffering on.
If you force triple buffering on, on the video card, this will cause problems as the game already does its own triple buffering for the video output.
You can easily swamp 512MB of video RAM with a combination of anti-aliasing, 2048 textures, and/or soft shadows or environment mapping enabled in conjunction with bump mapping.
Easy to test these theories. Turn down everything and see if the stutters go away.