Scroller MX04 to Flash8 What is the problem?

I have a scroll bar that I adapted from a tutorial for a current project, it worked perfectly in FlashMX04, but I recently updated to Flash8 on the assumption that it would still work fine, but the problem is that it will not work in subsequent frames(for different content, as it did in MX04)

I have tried so many other options that it is not funny.

I’ve included the ActionScript below, the FLA here: http://thinkmonkeymedia.com/test/SimplestScroller.zip
and the SWF here:
http://thinkmonkeymedia.com/test/SimplestScroller.html.

Thank-you in advance.

// Defines Scroll Area
top = _root.movMask._y;
bottom = _root.movMask._y+_root.movMask._height-_root.movScroller._height;
left = _root.movMask._x;
right = _root.movMask._x+_root.movMask._width;
// Brings all objects to the Scroll Area
_root.movIn._y = top;
_root.movIn._x = left;
_root.movScroller._y = top;
_root.movScroller._x = right;
// When the Scroller is Dragged
_root.movScroller.onMouseDown = function() {
    if (hitTest(_root._xmouse, _root._ymouse)) {
        startDrag("_root.movScroller", false, right, top, right, bottom);
    }
};
// when the Scroller is Released
_root.movScroller.onMouseUp = function() {
    stopDrag();
};
// Update Position every 10 milliseconds
setInterval(Update, 10);
function Update() {
    var ratio = (_root.movIn._height-_root.movMask._height)/(_root.movMask._height-_root.movScroller._height);
    _root.movIn._y = ((_root.movMask._y-_root.movScroller._y)*ratio)+_root.movMask._y;
}