extremely low fps in fb caves

PW runs fine outside of caves, i can easily do 30/40+ fps in most areas, excluding cities (especially Archosaur <_< ). However, once i enter a cave i am lucky to get 10-15 fps, maybe 20 tops at times.

As far as i can tell it is something to do with the terrain (floor); if i look at the walls or ceiling i get a playable framerate. also, if i move the camera so that it is pretty much on the ground (but looking at the same scene), my framerate rises considerably, and i'm guessing this has to do with the occlusion culling technique used on the terrain mesh, which also seems to point to it as being a terrain problem, maybe even a bug.

some info about my machine:
nvidia geforce 5200 (128 mb)
512 mb ram
P4 2.2ghz

I am currently running the game with everything on lowest settings and at 1024x768. I can achieve a barely playable framerate in caves with 800x600 resolution, but the UI clutters the play area immensely.

i have also used nHancer to tweak the game somewhat, but to little effect.

anyone have any suggestions aside from upgrading my hardware (most likely the cause of the problem) ? is this a common problem? if you happen to have similar specs to mine, do you also experience this problem?
thanks.