Well that may be because whatever vectors you're using are too dense and the processor can't keep up with the art so you might try simplifying the art. One way you can see if there's a lot of density to the art is to select the Subselection tool on the toolbar (the white arrow) and select your art. If it has a lot of points on it (the little dots) your art might be too dense to display at the correct frame rate. Or if you've made a movie clip out of the thing you're doing it might be because 20fps is an odd rate and your computer or software you're using to look at the film just can't process it correctly. Standard film is 24fps or 30fps so you might ry changing the frame rate to a more normal one.