Skyrim keeps underclocking my GPU after a couple hours play, when i'm in game it suddenly drops to about 15-25FPS and when i chek the clocks on my gpu it is way lower than normal, over half. I know this isn't overheating because i keep GPU-Z on to monitor temp in game and it never exceds 70°C, i have also tried playing the game with low graphics and it's the same. The only way to fix it is restarting my computer, which is very annoying to do over and over, this had never happened before Skyrim. Please help

Specs:
CPU: intel i7 2600k
GPU: Geforce GTX 560ti
RAM: 16gb DDR3
OS: Windows 7 64bit
SSD hard drive

Do you happen to have shadows at ultra or somewhere like that? Since the cpu is doing all the shadow work, the gpu is constantly left unchallenged and finally decides to clock down in order to save power. This probably results in even less fps.

That's my idea.