Continous Circle Tutorial Problems

i follow the tuts… and try to customized…

i set my movie to 850 by 200 adjust the this_x=-1500

when i first load the movie… its cool… but if i move the mouse up / down…or just wait without moving it… the circles passes a few time…but then it becomes a blank white screen… meaning…the circles dont respawn back and repeat…etc…

what did i do wrong?