Tweens jamming half way


I have an array of movie clips.
One is added to the stage.
The user enters a value in a text field.
The displayed MC is tweened off the stage and the next MC is placed onto the stage.

for some reason the tweens appear to randomly jam half way. it happens about 1/10 of the time.

My code is probably too long to post but the most relevant bits (I think) are:

I create my instances of display_mc as below.

 tempMC1 = new display_mc;

When the user enters in some data (usually a two letter answer) this runs:

 private function moveMC(movingMC:DisplayObject,begin:uint,finish:uint):void
 {//shrinks MC's off to the right.
  moveTween = new Tween(movingMC,"z",None.easeOut,begin,finish,3,true)

could it be because I use the same var name for the tweens?
could it be because I use the same var name for the MCs?
could it be because I have my MCs running through frames that tween their colour at the same time?

I have spent the better part of 3 days trying to trouble shoot this code and I am plum out of ideas. Any suggestions would be greatly appreciated.

if you want to view the swf it can be found here:
(it loads alot of images via XML)

Thanks heaps for any help!
As always very much appreciated.