i had the same issue a while back, where the framerate would randomly drop dramatically. I had had this issue in bf3 aswell, and i finally solved for this christmas!
I have changed alot in my computer for the last year or so, hoping to make it stop (hd 7850 to 280x, phenom 965 be to fx 6300), but the problem kept on being apparent whenever i would start bf or farcry 3.
I turned out, that i could stop these drops, by cooling my mobo (between the socket and pci e slot) in an ekstreme fashion, and this made me realize, that it was my motherboard which was the problem
i therefore got the m5a99x, and there is absolutely no problems now!!
i dont know if we are experiencing the same problems, but it is worth the try!
at the moment i am extremely pleased, and i realy hope, that this helped you!