Constant rate of motion via script

I’m tyring to get a movie clip to move from it’s current x, y
position to a designated x., y postion specified
in a button (onrelease).
But I need the movie clip to move at a constant rate regardless
of it’s distance traveled.
I can’t get it to work.
I’m having troble knowing which code to put where,
Button, object and frame.
Can anyone help?

code for the button:

on (press){
   pathToTheMovieclip.gotoAndStop (2);
}
on (release){
   pathToTheMovieclip.gotoAndStop (1);
}

[SIZE=1]note: the pathToTheMovieclip should look like that (example of a mc called ball on the main timeline) _root.ball[/SIZE]

On the mc:

1- create two identical frames;
2- on the second one place this code:

this.onEnterFrame = function(){
	speed=3;
	this._y+=speed;
}

[SIZE=1]note: speed variable could be set to any value you want, the _y is for vertical movement, if you want to move horizontally use _x[/SIZE]

=)