QuickTime playback jitter

I have tested the quartz composition on a PowerMac G5 quadcore 2.5GHz tower with 2GB RAM and an nvidia 6600 card as well one with a QuadroFX card (same spec-ed computer), and a MacPro Quad Core machine (brand new) highly spec-ed up. 

Using Quartz we need to simultaneously play a soundfile (.wav) using the following plugin: http://www.qcplugins.com/

All of them seem to play back QuickTime movies of this size with some jitter - whereas QuickTime Player handles the movies perfectly. We can simulate the playback jitter in QTPlayer by using "play all frames."

I have tried various codecs to improve playback including: Apple Intermediate, H.264 and finally PhotoJPEG (at 75% quality) - the final one being a spatial-only compression seems to make a slight improvement - it eliminated dataspikes in playback, but performance still varies between 60-17fps. Even restricting playback from Unlimited to 50Hz made little effect.

Are there any tricks/workarounds in Quartz to resolve this issue or is it something to do with the way Quartz handles the timebase and playback that is unavoidable? I assume that Quicktime is caching the video, whereas Quartz isn't. Even with saving the movies with "preload this track" and "cache hint" the QuartzComposer playback has not improved. We have tried using both patchtime and systemtime to synchronize video and audio, but this doesn't seem to make any difference. The Quartz Composition is randomly loading 30sec movies from a large selection of movies held in different folders.

any help much appreciated.