onRelease stop function? help!

Hey guys, i have a pressing issue here was wondering if anyone could help me with.

I have a small movie that basically is a mask that when you click on it it follows the mouse to reveal the other image under it. I dont want a circle bit of a mask, i want the mask to be 1/2 the stage size and only scroll on the x plane. I have this setup and working more less except for the fact i cant “uncall” (yes i just made that up) the function. heres my code

var speed = 5;
MovieClip.prototype.follow = function() {
this.onEnterFrame = function() {
this._x += (_root._xmouse-this._x)/speed;
if (Math.abs(_root._xmouse-this._x)<1 && Math.abs(_root._ymouse-this._y)<1) {
delete this.onEnterFrame;
//drag the mask
mask.onPress = function() {

as you can see the mask when pressed gets the follow function attatched, but i cant get it to stop moving when you release. I have tried all manners of onRelease to no avail…what can i do?

I would use the startDrag function but i love the easing (and i dont know how to constrain start/stopDrag to the X axis)

Any help would be great!