Ok,

I have had Skyrim since the release day and stopped playing the game for a while in around Mid February during this time I had seen no FPS drops. However after coming back to Skyrim in this month I have been experiencing FPS drops after playing for about 1 hour and a half. My FPS is usually always at the 60 FPS mark. But when the fps drop hits it stays at around the 35 FPS mark and starts when I either fast travel or enter a building and stays at around 35 FPS and doesn't go until I restart the game or alt tab out of the game and back in. I have tried different Nvidia drivers to solve the problem but didn't fix the issue. I have also checked my GPU and CPU Temps and they are at good temps. Anyone know what is wrong here?

Specs:

Nvidia Geforce 560ti
4GB Ram
2500k 3.3Ghz
Windows 7
1TB Hdd

Anyone know what the problem could be?

Sounds like exhaustion of VRAM, happened to me on my old 1.5gb 580. Try lowering some settings like AA or AF or shadow quality, or upgrade to a higher vram 3d card.