ActionScript 1/2 :: Repeat Function After X Seconds?

I just wonder if I could make the object "car" to repeat doing this:
 
function start_car(){
_root.car.onEnterFrame = function(){
_root.car._y +=15;
};
};
setTimeout(start_car, 1000);
 
I want it to begin at y=0 and x=0 after like 7 seconds and then repeat going down the y axis as you can see in the code.
 
So the car movieclip starts moving slowly from the top going down then  after like 7 seconds when it's out of the scene it begins at e.g x=0 y=0  and going down for 7 seconds then starts at x=0 y=0.... etc, so it  keeps looping.