Two onEnterFrame commands (in separate frames)

I have two onEnterFrame functions that run at separate times. The code is on the timeline of a mc that I am using as a mask, and it is controlling an empty mc on my main timeline that is holding a jpeg loaded into my main movie using the loadMovie command.

On the 35th frame of the mask mc timeline, I have this code (to scroll the picture)

_root.holder.onEnterFrame = function() {
_root.holder._x += 3;
}

It works fine until frame 503, where I have this code (to fake that the masking is being used to fade the picture in, I am setting picture alpha to zero, then fading it in)

_root.holder._alpha = 0;
MovieClip.prototype.fadeIn = function(speed){
this.onEnterFrame = function(){
this._alpha+=speed;
if(this._alpha >= 100) delete this.onEnterFrame;
}
}
MovieClip.prototype.fadeOut = function(speed){
this.onEnterFrame = function(){
this._alpha-=speed;
if(this._alpha <= 0) delete this.onEnterFrame;
}
}
_root.holder.onEnterFrame = function() {
_root.holder.fadeIn(5);
}

How would I get the picture to keep scrolling? (because it stops once the new onEnterFrame function is executed)