ActionScript 2.0 :: Moves A Movieclip To The Opposite Y Position Of The Mouse Pointer?

I have a basic script that moves a movieclip to the opposite Y position of the mouse pointer:

Code:
var cP = this.createEmptyMovieClip("centerPoint", 1);
cP._x = Stage.width / 2;
cP._y = Stage.height / 2;
var myBox = cP.attachMovie("boxMC", "myBox", 2, {_y:cP._y});
cP.onMouseMove = function() {
myBox._y = (cP._ymouse * -0.9);
};

Everything works great, but I'd really love to be able to ease the box to it's Y position, rather than moving it to it's final position immediately. I'm having a difficult time with it because the box is moved based on the mouse position, which obviously doesn't ease.