My current settings are

1920x1080 fullscreen
2x aa
filtering disabled
high textures
medium blur
high shadows
medium decals
fxaa enabled
full water detail

And the game will run fine (high 50s-60 fps constant) for hours, but occasionally when either entering an interior, leaving an interior, or leaving a town (and only during these three circumstances, ie not when entering a town) my fps will drop below 10, and will persist until the game is restarted. Any fix for this? Ask if you need any additional info.