[wvxvw]

var myText:String = "My very long text";
var counter:Number = 0;
_root.onEnterFrame = function(){
_root.my_txt.text = _root.myText.substring(_root.counter, _root.counter+5);
if(_root.counter>=_root.myText.length){
_root.counter = 0;
} else {
_root.counter++;
}
}