What is the specification of your computer and Windows version?
What isth source and type of video, is it HD?
You can also use the Search box on the right - search for 'jerky playback' or 'stutter playback' - you will find that this question has been asked many times before.
I agree with John, there are so many things that can affect the situation that you describe that without more information any one of us would be shooting in the dark and we would give you pages of things to try and they may still not answer your question. So for example, I could say, did you preview render the video, did you change the playback resolution (depending on your version of MEP), are you using processor intensive video file packages, how much ram do you have, what processor are you using, 64 bit or 32 bit, which windows OS are you using, as John said, are you editing HD videos, are you creating a 3D video, how big are the files that you are using, how long is the movie, ....the questions could go on and on and each or anyone of the answers to those questions could make it much easier to help us zero in on the problem and give you a realistic solution.