almost certainly user error is the problem. attach a screenshot of your timeline.
also, i'm dubious that timeline tweening will run more efficiently than as tweening. what makes you think that?
I found the problem - the movie clip is positioned by the user entering the X and Y positions in an XML file, and when the position was set it seemed to conflict with the timeline tween - removing the positioning code made the tween run just fine.
I did end up using the AS tween, it's just that we use very, very specialised hardware and there was no precedent for AS tweening being used, we try to alleviate the load on the CPU wherever possible. Thankfully it runs without any issue.