Random MovieClips help

How do I continue a smooth fade in each time a random movie clip loads into the ‘cover_mc’?

var covers:Array = new Array("usa","uk","germs");


function coverchange(){
    var i:Number = Math.floor(Math.random()*3);
    onEnterFrame = function(){
    cover_mc.newCover._alpha +=10;

each time the new mc loads in, it obviously wipes out the previous attached movie, creating a moment of blankness …