Help with Nav

Ok, I’ll try to do my best to explain this. I also attached a fla file just in case.

I have three MC’s called nav_mc, menu_mc and experiment_mc. When I rollOVer the menu_mc, the nav_mc is called and takes place of the menu_mc. Inside the nav_mc is the experiment_mc (which is a button). When I click on experiment_mc, the menu_mc is called back into place and the nav_mc fades away. My problem is when I rollOver the menu_mc again, the nav_mc doesn’t re-appear.

Any help will be greatly appreciated as I’m new to flash and this is all a great learning experience.

nav_mc.onEnterFrame = function() {
if (this._alpha>0) {
this._alpha -= 5;
} else {
delete this.onEnterFrame;