I found that while playing Skyrim my game started to lag horribly, even on lower settings than the game itself recommended. I accidentally discovered this by bringing up the Steam Overlay to chat with a friend. I then alt tabbed by accident, got back in, and suddenly the game runs at constant 60FPS, even at ULTRA settings.

However, it seems that not only is it the Steam Overlay causing this, but also the ingame menus, such as when you loot a box or browse your inventory. You have to alt tab out and back in each time to restore your game back to stable FPS. I hope this is addressed in patch soon. I've not tried Window Mode however, this only goes for Fullscreen. Can't be arsed to try it out yet.

Edit: It seems that it is NOT caused by the ingame menus, but only the Steam Overlay after all. Still, just alt tab out and back in if you've brought it up to fix this it seems.