Why is this so "glitchy"? (AS2)

1

http://monkeyheadgraphics.com/theLab/flash/vegan/

i’m able to get this to work but it feels so sluggish…
here is my script:

mouseListener.onMouseMove = function(){
if(_root._xmouse<= 150){
var meunTween:Object = new Tween(navMain, “_x”, Elastic.easeOut, navMain._x, -50, 1, true);
updateAfterEvent();
}
if(_root._xmouse>= 150){
var meunTween:Object = new Tween(navMain, “_x”, Elastic.easeOut, navMain._x, -300, 1, true);
updateAfterEvent();
}
}

thanks,
Andre