Hi, here are some more details.
The 50fps photojpeg QT is playing from the internal SATA HD.
The animation is of a sphere travelling accross screen.
Activity monitor shows quicktime process is using 35% CPU.
QT Datarate is 5.6 Mbits/s
Dual 2 GHz Power PC
1.5 GB Ram
Geforce 6600LE video card
Dell 2407 monitor
Changing display frequency from 60Hz to 75Hz
alters the appearance of the playback but it is still jerky.
Playing the animation in a HD video program onto a video monitor results in smooth playback.
Cheers
Adam