Arena mode kinda stuttery

[The MiMiC]

Playing the regular game for Sin Episodes, it runs pretty good for the most part, except in the dock area (which I think slows down for pretty much everyone).
But, Arena mode seems to run at a pretty low frame rate in all the maps I've tried so far. My favorite is the one of the entry hall. But, if I go down the stairs into the center of the room, pretty severe framerate drop. So, I'm pretty much limited to circling the outside of the map, going up and down stairs, and if I even look through the window towards the room I get the same framerate drop again. Playing in the office map, with all the cubicles, is even worse.
I had thought at first it was the demo recording doing it and so disabled it. No improvement. Then I tried playing at 1024x768 with all settings at minimum for graphics, no change. Any other suggestions?

PC is 2.53Ghz P4, 1GB ram, XP home SP2, Nvidia 6800GT w/256MB, page/swap file set to max size.