Try this:
var timer:uint;
function btn_over(event:MouseEvent) {
timer = setInterval ( function () {
hover_bubble.x = itemX + itemW + 10;
hover_bubble.y = itemY + 30;
hover_bubble.gotoAndStop('max');
clearInterval(timer);
}, 500);
}
function btn_out(event:MouseEvent) {
clearInterval(timer);
event.target.gotoAndPlay('out');
hover_bubble.gotoAndStop(1);
}