I was in Falkreath, at night, and it was raining. I have all settings maxed out, at 1920x1080 res. For some reason my fps was going down to as low as 18fps occasionally. I also have no graphics mods installed, including the free texture dlc.

i5 3450 @ 3.1Ghz

HD 7870 2GB

8GB DDR3-1600

Why is this happening?