Sliding images puzzle "game" - Help!

Greetings,

The “game” I need to make consists of 1 large (vector) image split up into 4 rows. The user is to arrange each row (one at a time only, top to bottom) so that the picture is completed. Upon each row completion, a message is displayed and the user moves on to the next row.

I have the basics down in Flash, and I can move the image pieces horizontally with keyboard arrows. The only thing I need help with is how to specify when each picture slice hits a certain X coordinate to “goto” a keyframe, load a movie, whatever need be…

The code I have thus far:

watchKeyBoard = new Object();
watchKeyBoard.onKeyDown = function() {
trace("a key is pressed. the key that is pressed is "+Key.getAscii());
};
Key.addListener(watchKeyBoard);
function moveStuff() {
if (Key.isDown(Key.LEFT)) {
this._x -= 5;
}
if (Key.isDown(Key.RIGHT)) {
this._x += 5;
}
}
box_mc.onEnterFrame = moveStuff;