Quicktime movies stop, start, stop

I have a long running problem with mpeg and mov files in quicktime. Although I have a fast machine and the latest (not pro) version of Quicktime, I frequently have trouble playing movies without the image pausing (while the location bar keeps moving) and then starting up again, then pausing.

This happens even with all other apps quit. I have a dual 2.3 G5 with 1 Gig memory. Should be enough, no?
Are you playing movies via Internet (from a web location) or stored locally.

If they're stored locally, are they on your hard drive? An external hard drive or USB memory device? A CD or DVD? in many cases when this happens it is caused by the media or the connection to the media not being fast enough to keep up with the movie.

What format are the movies in? If you go into QuickTime's "Movie Properties" what codec does it show that you are using?