Every time i play skyrim it runs perfectly fine for the first 30-60 minutes. Then when ever i go through a door or fast travel anywhere my FPS drops for a solid 60 to about 15. This has just started to happen too. I don't know why or what is wrong but if anyone has any ideas on how to fix this i would really appreciate.

Here are my specs...

AMD phenom II X6 1055T @ 2.80 GHz
8 GB Ram
Geforce GTX 550 Ti