Problem with Dynamic Text fields

Hello, I have problem and I think its simply but I cant do anything....

when I have this:

var container_:MovieClip = new MovieClip();
        this.addChild(container_);

var movie_clip:hello = new hello;

function add_my_text(){

     container_.addChild(movie_clip);

     movie_clip.my_text.text = "hello";

}

BUT IF I HAVE LIKE THIS:

var container_:MovieClip = new MovieClip();
        this.addChild(container_);

var movie_clip:hello;

function add_my_text(){

     movie_clip = new hello;

     container_.addChild(movie_clip);

     movie_clip.my_text.text = "hello";

}

I get the error.  TypeError: Error #1010: A term is undefined and has no properties. The same with EventListeners and etc. Why this hapening?

if I write my movieclips in frames like for example:

var movie_clip:movieclip = new movieclip;

My flash is going very slow becouse I have many movieclips in library.

Thanks.