Flash MX inconsistancy

Hi all,
First off, im extremely frustrated with flash. Just had to get that off my chest. Here is my problem.

I am simply trying to use actionscript to move a movie clip on the y axis. I have got it to move so far but the end result varies. When I move the clip further down on the stage it seems the clip ends further up than I want it. I tried putting in the x/y cordinates exactly yet it ignores it.

Here is my code. BTW, if I increase the speed variable to say 40/50 or 100, it moves the movieclip higher and higher on the stageā€¦ I added the file as an attachment to this post.

onClipEvent (load) {
speed = 100;
endat = 210;
this._x = 379;
this._y = 280;
}
onClipEvent (enterFrame) {
if (this._y > endat) {
this._y -= speed;
} else {
boxbg.stop ();
}
}