_rotation causes gfx problems

Hi im tryin to rotate this movieclip of a starfield but when i use _rotation i get this flickering effect every second or two of rotation
see
http://www.mn001c7645.pwp.blueyonder.co.uk/lightforcerotate.html
left and right keys change rotation
hold down a key and you will see what i mean ,is there a way to prevent this flickering
here is the code i used

 onClipEvent(load)
{
 speed = 0.001;
 angle ++;
}
 
onClipEvent(enterFrame)
{
 if (Key.isDown(Key.UP))
 {
  xx = math.cos(_rotation*Math.PI/180)*speed;
  yy = math.sin(_rotation*Math.PI/180)*speed;
  this._x += xx;
  this._y += yy;
 }
 if (Key.isDown(Key.LEFT))
 {
  this._rotation -= angle;
  key_down = 1;
 }
 else if (Key.isDown(Key.RIGHT))
 {
  key_down = 1;
  this._rotation += angle;
 }
}
 

what is the particle emission code ?

Bill