First, try transferring a movie from the stick to the hard drive to see if it plays better.

You still haven't mentioned what file format you're trying to play.

Overclocking:

http://homepage.mac.com/schrier/mhz.html

http://lowendmac.com/macdan/02/0624ek.html

http://www.colino.net/geek/powerbook/oc-lombard.html