Slidebars and movie clips

I have been working on a simple idea for controlling the timeline of a movie clip with a slider, that is place on the root timeline. I have attached a simple fla that sort of works except. However there are two main problems. You have to return the slider to its start location to update the change of position and also I can only get it to work by looping over two frames.
Dose anyone have any ideas.

Here is the code

fadeBox_mc.onEnterFrame=function(){
if(fade){
this.nextFrame();
}else{this.prevFrame()
}
};

slider = _root.mySlider_mc.Knob._x;
onEnterFrame = function() {
if (slider > _x){
_root.fade = true;
}else{
_root.fade = false;

}
};