globalToLocal problems

I'm trying to make a zoomable/pannable map in Flash that will eventually be used on a kiosk with a joystick.

I'm simulating this behavior right now with the mouse wheel and it works fine to a degree until I stop using mouse coordinates and start using the coordinates of a custom "cursor" movieclip.

http://wonderfl.net/c/1L5I

You can take a look at the code and working sample there. When I try to pass in the coordinates of the cursor relative to the Map clip, it just gets super jumpy when zooming in, moving the cursor, then zooming out.

I'm trying globalToLocal, but that appears to give me jumpy results as well. Shouldn't a myMovieClip.globalToLocal operation on the root mouse coordinates give the same numbers as myMovieClip.mouseX/mouseY? Because they don't.

Any insight?