Rising movieclips - reset position

I have a moviclip duplicating 200 times and rising off the top of the screen. When it moves completely off the stage, and I want it to move back to the bottom of the screen and rise up again. Right now it just keeps going up off the stage and doesn’t replace itself. Any ideas? Here is my code on my movieclip:

onClipEvent (load) {
screenwidth = 400;
screenheight = 400;
speed = (Math.random()*1)+.1;
_y = Math.random()*screenheight;
_x = Math.random()screenwidth;
}
onClipEvent (enterFrame) {
_y -= speed;
//if brick hits height limit reset brick to top of screen
if (_y>(screenheight+(_height/2))) {
_y = 0-(_height/2);
//give new x value within in screen
_x = (Math.random()
(screenwidth-_width))+_width/2;
}
}

Here is the code on my actions layer:
fscommand(“allowscale”,“false”);
fscommand(“fullscreen”,“false”);
stop();
//tell how many clips
clipcopies = 200;
//duplicate clip however many times we specified above
for(var v=0;v< clipcopies;v++){
smoke.duplicateMovieClip(smoke+v,v);
};

Thanks!