I get this problem too. If you click on "Clip" instead of "Movie" and then watch it in the window it should be ok - well it works for me.
You could try producing your project to a file and then play it to see if the problem is still there in the finished product or if it's just an issue with the preview window.
Something else you could try is lower the quality of your preview window to see if that helps your computer keep up.