actionscript 3 - How to random every time object.Y is out of stage -

im trying random object(2) every time object(1).y beyond stage. problem comes moving , "jumps" position im looking make change. did try "if (road1.y >= stage.stageheight) {" doesnt trigger. , when i'm doing the speed of movement triggers when has been on stage 2 times before. registration point of movieclips in top left. code this

private var road1,road4:road1;     private var road2:road2;     private var road3:road3;     private var randomroad:sprite = new sprite();     private var offset:number = 0;     private var counter:number = 0;   public function onadded(e:event):void {         removeeventlistener(event.added_to_stage,onadded);         addchild(road1=new road1());         addchild(randomroad);         addchild(road4=new road1());         addeventlistener(event.enter_frame, onenterframe);     }      public function onenterframe(e:event):void {           if (startrandom == true) {             if (math.random() > 0.5) {                 randomroad.addchild(road2 =new road2());             } else {                 randomroad.addchild(road3 =new road3());             }             startrandom = false;             trace(randomroad);             trace(startrandom);         }          if (road1.y >= stage.stageheight) {             startrandom = true;             trace("zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz");         }          offset +=  speed;         counter++;         road1.y = offset % 800 - 800;         randomroad.y = road1.y + road1.height;         road4.y = randomroad.y + randomroad.height;      } 

try this:

if (road1.y >= -speed) {     startrandom = true;     trace("zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"); }