If it has only just started doing it ..
a) If you just installed a new add-on recently and then it started stuttering...try removing what you just added
b) Something else could be wrong. Have you changed ANYTHNG else in your system, or have any other programs started to show signs of stuttering or slowdown?
Usually, a game causes problems or it doesn't....when it suddenly starts causing problems when nothing else has changed after previousley running fine, you probably need to look elsewhere for the cause.