I do manage 90 - 100 fps, as the game goes on though, I slowly start to lose fps. I took note of what you said "Your CPU shouldn't be too bad, so you should reach at least 60 fps when no action is happening." and kept and eye on my FPS In and out of active area's and seen no drops in fps early into a match. Maybe this is some kind of a memory leak issue that you are unaware of, because I shouldn't have 10 - 15 FPS by the end of the game.
Edit: I'm no tech genius, but I get better performance in the new Call of Duty then this game.