Scroll bar moves when clicked on

Hello.  I have a custom scroll bar that was made using AS2.  It works fine except that when you click on the bar and move it, the image shifts slightly and then doesn't line up correctly.  I don't know what's causing it to shift.  All registration points are in the upper left hand corner.  I was really hoping someone could help me with this.  Here is the code:

txt.setMask(mask)

scrollbar.onMouseDown = function() {

    if (this.hitTest(_root._xmouse, _root._ymouse) && txt._height>mask._height) {

        this.startDrag(false, scrollbarBG._x, scrollbarBG._y, scrollbarBG._x, scrollbarBG._height-this._height)

        txt.onEnterFrame = scrollThumbs;

        dragging = true

    }

};

scrollbar.onMouseUp = function() {

    stopDrag()

    dragging = false

    delete this.onEnterFrame;

};

function scrollThumbs() {

    var funkyVar = -this._parent.scrollbar._y*(((this._height-this._parent.scrollbar._height)/(this._parent.scrollbarBG._height-this._parent.scrollbar._height))-1)

    this.Y = (funkyVar-this._y)*.2;

    this._y += this.Y;

    if(Math.abs(funkyVar-this._y)<1 && !dragging){

        delete this.onEnterFrame

    }

}