I normally don't have too much problems with FPS - I have a bit of jerkiness when I pan and run at the same time, and that's something I've been trying to fix, though so far without success. However what really bothers me is that occasionally (as in once everytime I play), I'd walk out of an interior area into Skyrim and my FPS drops to a crippling 10. These can be areas I've visited many times before without issue. The last time it happened was when I walked out of a building in Riften on an overcast day. In fact, it seems to be overcast more often than not when it happens. Re-entering a building returns my FPS to normal, but exiting again gives me the same problem. Reloading without quiting to desktop doesn't fix the issue. However, when I quite to destop and then reload it's normal again. I never had this issue with the early versions of Skyrim. Any ideas?

Video card is Nvidia GeForce 460.

I have triple buffering in effect (though I'm skeptical if it actually improves my performance), a few texture mods, but no lighting mods. I haven't found any performance enhancing mods that actually improve my overall performance either. I've optimized my settings as best I think I'm able, though I haven't made any permanent changes to the .ini files.