Interactive Background wobbles

1

…when it lines up with the mouse. Any ideas why? http://www.gregrochford.co.uk/kirupa/bar.swf
http://www.gregrochford.co.uk/kirupa/bar.fla

onMouseMove = function () {
    constrainedMove(bg_mc, 4, 0, _xmouse);
};
function constrainedMove(target:MovieClip, speed:Number, dir:Number) {
    var mousePercent:Number = _xmouse/Stage.width;
    var mSpeed:Number;
    if (dir == 1) {
        mSpeed = 1-mousePercent;
    } else {
        mSpeed = mousePercent;
    }
    target.destX = Math.round(-((target._width-Stage.width)*mSpeed));
    target.onEnterFrame = function() {
        if (target._x == target.destX) {
            target.onEnterFrame = null;
        } else if ((target.destX-target._x)>0) {
            target._x += Math.ceil((target.destX-target._x)*(speed/100));
        } else {
            target._x += Math.floor((target.destX-target._x)*(speed/100));
        }
    };
}
function definedMove(target:MovieClip, speed:Number, destination:Number) {
    target.onEnterFrame = function() {
        if (target._x == destination) {
            target.onEnterFrame = null;
        } else if ((destination-target._x)>0) {
            target._x += Math.ceil((destination-target._x)*(speed/100));
        } else {
            target._x += Math.floor((destination-target._x)*(speed/100));
        }
    };
}