I had the same issue when I was using the Catalyst 9.3 drivers from ATI. I am running BnW2 with latest patch on Vista 64. ATI 4870x2 and 4gb Ram. I messed with several different card settings...nothing changed it. But, I found that if I saved my level and then reloaded it, the problem went away. This was only a temporary fix, however, because it would occur again next time I ran the game.
The permanent solution for me was to completely uninstall Cat 9.3 and reinstall 9.2. In addition, I discovered 9.3 caused bad flickering on transparent textures, UI overlays and distant backgrounds on all sorts of different games. Hope 9.4 fixes these issues.
I hope this helps you.