We would need to know more about your system specs before we can help you. FPS drops are usually caused by one piece of hardware being unable to keep up with the rest of the system, usually either graphics card or CPU. Video driver problems can also cause framerate drops, same with insufficient amounts of RAM.
Without knowing more about your system, I can only suggest that you go to your graphics card manufacturer's website and download the latest drivers for your video card. If your framerate drops are caused by a driver problem, that should fix it.