You may be experiencing slow data transfer. If you pause the movie and wait for more (or all) of it to download before you resume playing, it will play without stopping.
You can tell how much of the video has downloaded by looking at the controller bar at the bottom of the player window. You will see the bar fill in with gray, from left to right, as the video downloads. Once the controller bar is completely filled in with gray, the video has finished downloading.
If you’re having problems that might be related to internet speed, try: