Playing ripped DVDs jumpy from USB drive

Hi,

I am playing ripped DVDs … they are very jumpy. It seems like a disk drive/transfer speed things as if I pause for a few secs and hit play it plays smoothly.

Any hints to remove this???..

I only have a 40GB internal drive, so no chance of copying all movies to the internal appleTV drive. I am using a unpowerd USB hub.

regards

Adam