ActionScript 3.0 :: Photo Gallery - Sliding Thumbs Reverse Button?

I'm trying to make a site with a photogallery that has sliding thumbs in the bottom corner ("slidingOut_mc"). I've got them sliding from right to left with a simple tween that is infinite. The problem is that I have a left arrow and right arrow that I want to slide the thumbs in reverse (left arrow i.e. "back_btn") or at double speed (right arrow i.e. "forward_btn") on a roll over of the button. I used the ENTER_FRAME thing to get the mc to reverse with prevFrame when rolling over the "back_btn" but I can't get it to stop when I roll out.

This is what I have so far...
back_btn.addEventListener(MouseEvent.ROLL_OVER, backF);
function backF(event:MouseEvent):void{
slidingOut_mc.addEventListener(Event.ENTER_FRAME, everyFrame);
function everyFrame(event:Event):void{
slidingOut_mc.prevFrame();
[Code] .....

It tells me the "everyFrame" is an undefined property. I think and hope the problem should be obvious to those who know more than me. Do I need some sort of import at the very start of it all?