The sites themselves might not be the cause of those jerky videos.
The first culprit might be your internet connection. The bandwidth might not be enough for the particular video stream. It could be that there are several computers connected at the same time to the same router.
It could also be your laptop. The hardware might not be fast enough to play the video that is being streamed to you.
If this is the case, lowering the video quality will help in making the movie play a little smoother.