I don't understand which for me is not uncommon.
When i used a (external) FireWire DVD drive the video playback of a commercial DVD movie was smoooooooth.
When i used the same FW DVD drive running in USB mode the same DVD movie video playback was jjjjeeeetttterrry.
Then tried a USB only DVD drive and the video playback would play back in spurts. Funny and scarry to watch.
Tested this on a G4 and also my beefed up 8500/SonnetG3 and had the same results. That is, FW DVD drives was the way to go.
So as i said i may have misunderstood your claim.