Hi Mouthy,
What is happening is that not enough of the video can load on your computer to give you a smooth viewing experience. One way around that is to go fix yourself a nice cup of tea after you begin the download. Take your time with the tea -- maybe read a bit of a book you enjoy while you're having the tea, or, you may choose to knit a couple of sweaters -- when you come back to the computer, the movie may have finished loading and playing. Then you can hit "replay" or "play again" or whatever it is called, and it should stream properly, since now it is all loaded up on your computer.
I hope this is helpful.