If you have recently upgraded to QuickTime 7.0.4, you may (or may not) have noticed that some videos that used to play fine no longer do so. If you're getting choppy audio and/or choppy and laggy video playback after upgrading, I've found a simple fix.
Go into QuickTime Player's Preferences, and turn off the option for "Use High Quality Video Setting When Available." You must re-open your videos to see the changes, but they should play much smoother now.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006011414591168