Tween Thumbnail Scrolling - trouble with button control

Hi all, I’m a newbie, totally stuck, and would GREATLY appreciate any help on this. All I’ve been trying to do is to move a string of thumbnails - thumbHolder_mc - incrementally (like every 386 pixels) left and right, controling it with arrow buttons. Code below works like a charm EXCEPT when I try to stop the tween motion on either end.
I tried everything - _visible = false, enabled = false, delete function, etc etc, nothing works. In the my script below, buttons get disabled, but never recover. Function never comes “back to life”, which is what I thought it was supposed to do:

stop();
//connect right and left arrow buttons to function:
abuttonright.onRelease = function() {
tweenRight();
}
abuttonleft.onRelease = function() {
tweenLeft();
}

//create incremental movement forward:
function tweenRight() {
easeType = mx.transitions.easing.Strong.easeOut;
var begin = thumbHolder_mc._x;
var end = thumbHolder_mc._x -=386;
var time = 1;
var mc = thumbHolder_mc;
Tween = new mx.transitions.Tween(mc, “_x”, easeType, begin, end, time, true);

//disable and dim button once the thumbnail reaches the end:
if (thumbHolder_mc._x < 140) {
delete abuttonright.onRelease;
abuttonright._alpha = 30;
}else{
abuttonright._alpha = 100;
}
}

//create incremental movement backward:
function tweenLeft() {
easeType = mx.transitions.easing.Strong.easeOut;
var begin = thumbHolder_mc._x;
var end = thumbHolder_mc._x +=386;
var time = 1;
var mc = thumbHolder_mc;
Tween = new mx.transitions.Tween(mc, “_x”, easeType, begin, end, time, true);

//disable and dim button once the thumbnail reaches the beginning:
if (thumbHolder_mc._x > 140) {
delete abuttonleft.onRelease;
abuttonleft._alpha = 30;
}else{
abuttonleft._alpha = 100;
}
}

Would totally appreciate anyone’s ideas out of this snag!