Easing movements on resize for full browser flash

Im having problems creating easing movemrnts on resize with a full browser flash web page. A great example of this is http://www.dopeawards.com/ but for some reason them only have the easing for the _y value. This is the code I am using. “cover” is the movie clip on stage that i want to have easing properties. but for some reason when I resize the movement is jittery and choppy but it is easing. What will make it a smooth movement?

cover._x = Stage.width / 2;
cover._y = Stage.height / 2;

var coverx = cover._x
var covery = cover._y

var stageL:Object = new Object();
stageL.onResize = function() {

new mx.transitions.Tween(cover, “_x”, mx.transitions.easing.Regular.easeOut, coverx,(Stage.width / 2), 1, true);

new mx.transitions.Tween(cover, “_y”, mx.transitions.easing.Regular.easeOut, coverx,(Stage.height / 2), 1, true);
}

Stage.addListener(stageL);

Please Help!!