[Thomas Weimer]

Do you see a little gray circle spinning to the left of the play head in motion. If you do then the clip is still rendering the background process. This happens automatically when you select optical flow and depending on how long your new clip will be determines the time it takes to render. I have a Intel Core Duo with 6gb of ram and it took 30 minutes to render out from a 10 second clip. Once it is completed it should play all the way through smoothly. Hope this helps.