I think it's the random number on the y, because its in an always block I think it is changing the y position every frame, hense the up and down movement,
You could just put the wrap around screen behaviour on the actor and then when exits the screen it would appear at the other side.
But if you want random starting hights it might be easier to kill the actor when it exits the screen the create it again from where it started, when created set y to random number.