Hi all:
I have also experienced this problem and right now I'm convinced that it is a bug, affecting only the mac version of quicktime (windows version is ok).
In my case, the problem occurs only when playng .mpg (mpeg-1) files, created with my Sony foto camera (Cybershot DSC S-60). Other types of video files works great. The same problem also happens when viewing .mpg files from within iTunes (last version). I'm not using Time Machine, so the problem is not related to it.
I made the following experiment and found something interesting (similar to Idgit's previous comments):
-open an mpg file in quicktime.
-then, with the file open, open the quicktime/preferences panel
-place the two windows (the player window and the preferences window) side by side , ie non overlapping each other
In this scenario, if I click on the player window to make it the active window, the playback of the .mpg movie is faulty, as previously described in this thread...
but...
if --after the start of the movie-- I click on the preferences window to make it the active window (instead of the player's window), then the playback is ABSOLUTELY normal.
This means that .mpg playback works fine only when the quicktime player's window IS NOT the active window.
This only happen on my macbook (received april 2008, leopard). Same files on Windows version of quicktime works flawlessly (using bootcamp and win XP, or using a standard windows PC)
Bye