Has anyone else experienced problems with video (incorporated within MX) moving out of syne with the sound track?

I have not been able to reliably play a synced clip through to the end. I'm using a Quicktime segment compressed with the Sorenson codex (our of Adobe Premiere). I've tried various frame rates (down to 12 fps). The clips are not large, maybe :30 sec to a minute with a frame size of 360 X 240. Audio quality is set failry low. It's playing off a 10K RPM hard drive on a decent computer. The audio moves more out of sync as the clip plays.

Is it just me? Any help or advice is welcome!