FPS Issues on all games

its not the fact that your framerate is dropping , 10 fps from 75 isnt that noticable , the problem is your frametime is increasing (the ammount of time a frame is being displayed) this will give a noticable and very annoying effect and can be caused..............as you have found out by faulty ram , if the number of milliseconds a frame is displayed suddenly doubles for whatever reason then reverts back to normal the effect is very similar to low fps

Whenever I have any problem with my computer, just do a fresh install of windows. I find it useful because then I don't have to fuss around with problems over again.