After playing skyrim for a couple hours straight my fps drops to about 15, i have no problem running the game in ultra, i get a constant 60fps normally, but suddenly it drops to 15 and i have to restart my computer for it to be fixed, doesn't work just quitting the game, also i tried setting to low graphics, and it's the same, after couple hours 15fps, anyone know what this is?

-Kristján

Did u check your fan ? Did you clean your pc ? Could easily be because it's overheating.

yes, i set fan manually to 90% on GPU and monitor temps on both CPU and GPU, they are both within safe limits

What GPU do you have?
Also, i get this problem on another game, APB: Reloaded, if you close Skyrim and just use your computer (web-browser or something) for about 3 mins, it should be normal again.
edit: a friend was having lag, told me it fixed after he turned off anistropic filtering and AA

i have Geforce GTX 560 ti SOC from Gigabyte, thanks i will try your suggestions, thank you