Simple ball game - sprite flicker

programming flash
1

ActionScript2 in CS3.

On the stage i hawe a red circle movieClip (instanceName: ball).
In frame 1 (stage timeline) i the code:[INDENT][COLOR=Blue]//ball speed
ball.dy = 10;

ball.onEnterFrame = function()
{
//move ball
ball._y += ball.dy;

//bounce
if(ball._y < 0 || ball._y > 380)
    ball.dy *= -1;

}[/COLOR]
[/INDENT]When I play/publish the movie - the ball flickers!
How do I fix it?

/Urban