I just noticed what hardware Bill Graham1 lists as his Mac--a Power Mac 7300 running Tiger. He has to be using something like XPostFacto to get it to run on this older Mac. In any case, even if this 7300 has a processor board upgrade, it's not designed to handle Quicktime 7 video at a decent speed. I'd be interested to hear Bill's experience with Quicktime 6.5.2, under OS 10.3.x on his 7300--did it play movies at a normal speed? If so, then that would confirm he's seeing the Quicktime 7 bug, which is actually real--check some of the other posts in the Quicktime section from people with G4 Macs that are more than fast enough to play movie files using Quicktime 7, but who are seeing this problem, where other people with the same Mac model aren't, which indicates it's some sort of bug. My Mac, a G4 AGP at 500 mhz, played MPEG-1 movie files just fine, under OS 10.3.x, using Quicktime 6.5.2, but when I installed OS 10.4, which requires Quicktime 7, the same movie files now play for a few seconds, then stop; if I hit the Quicktime Player window's pause button, then press it again, the video starts playing at a normal speed, but only after skipping some frames, and then plays again for only a few seconds. If I keep doing this, the whole video plays, but that's not how it's supposed to work. The same thing happens when you run Quicktime Player 6.5.2 under OS 10.4.x--you can't run the Quicktime 6.5.2 installer under OS 10.4.x, but you can run its Quicktime Player application if you copy it from a Mac that has 6.5.2 installed--but it doesn't help, indicating the problem is something in the Quicktime 7 extensions, or a problem with some OS file(s) that affect Quicktime playback, at least with MPEG-1 files--I haven't tried any MPEG-2 files.
If you play these same MPEG-1 files using VLC Media Player instead of Quicktime Player, they play with no pauses, on these same "slow" Macs. I don't think VLC relies on Quicktime software. This further implicates Quicktime 7 as the cause of the problem.