Poor video playback may be due to insufficient bandwidth. Here are some tips on how to improve the playback quality:
First, when the session is open make sure you are closed out of any other programs that are running flash.
Second, when the video is running make sure your webcam video is not running. If you are not speaking over the video I would also recommend temporarily turning off your microphone audio stream as well.
Third, make sure only one video is open at a time. In general try to limit the number of materials you pre-launch into separate tabs. Try only to launch a file from the Media Library when you are ready to use it, and close out of it when you are through with it.
Please let us know if these tips helped resolve you issue.