I've been playing the game for over a hundred hours since it launched last year without any problems. I've had this problem crop up just recently, and was hoping someone might have a suggestion. There have been no changes to hardware since launch, and I keep my video card drivers up to date.

I installed Dawnguard, and now at almost random intervals of area loads, the game will suddenly drop from 60+ frames down to 2. The behaviour is very strange.

It seems to happen more frequently with Breezehome in Whiterun. I will go inside, and the framerate will drop for about 15 seconds until I move upstairs where it sometimes corrects itself and bumps up to 60+. Either that, or going back outside speeds the framerate back up.

Sometimes after it speeds up again inside buildings, the framerate will drop when I go back outside. As soon as I open inventory, or the map... full framerate again until I close them and need to walk around again.

I have no mods apart from Dawnguard and the hi-res textures. I run an i7 920, and a gtx 560ti. The settings I play with is a modified "high." I turn off anti-aliasing, turn up Anisotropic to 16x. In advanced I turn everything to either "high" or "Ultra" and check all the "reflect" boxes. I don't touch any of the automatic settings for the sliders though.