I'm having this issue too. I'll start the game at 60fps, but over 10 or 20 minutes, I idle around 40fps. This is with settings at max. When I turn everything down I only get back up to, like, 45fps. It seems like I should easily be able to hit 60, especially with the settings all the way down like that. I have a GTX 670 and an i5 3570k. Furthermore, when my fps drops down to the low 40s, and I start a conversation, it jumps back up to perfect 60fps! But only for the duration of the conversation, then it's back to where it was before. What the heck? This is particularly annoying for a PC game like this, where you interact mainly with the mouse. Anything lower than 60, especially by 10-20fps, feels really sluggish and just makes me not want to play the game. :(
