Getting jittery playback of remixes on KDP

Hi ,
I've been generating remixes using the appendMediaEntry action via my own flex application. When I playback the remixes I get really bad jittering. If I move the seek bar to that entry however it plays normally....look here for an example:
http://174.143.151.19/wemash_test/test.html

on the second webcam clip (after 50 cent speaks) it just jitters and doesn't play smoothly at all.

Any ideas on why this would occur?

When I flatten the mix and view the flattened file the playback is completely normal.