I have the exact same problem. The first year I got my PC w/ r9 270x had 300 fps straight on csgo. Now I get 96. I don't know why It doesn't go above 100. It is like it is capped or something.
i was having the same problem.
Solved it by using DDU Display Driver Uninstaller Download version 184.108.40.206 and reinstalling the driver.
Also, i did a small overclock on my fx-6300, now i'm back to 200+ fps.