AS2. while loop, setInterval, clearInterval problem

[M4X]

I'm trying to get my code working so effectively the playhead moves backwards for a certain length of time (20 frames, 1 frame every 33 milliseconds). The setInterval function works when not inside the WHILE but I need it to run 20 times and then use the clearInterval function to stop it. I can't seem to figure out how to solve it. Currently it outputs 1 through to 20 and then "clear interval". Can someone please give me a hand? here is the code which is inside an onRelease button holder. Thanks to anyone with input.

var myInterval = setInterval(slow, 33);
var counter = 0
while (counter < 20) {

counter++;
trace(counter);

function slow (){
trace("slow function");
prevFrame();
}
}
clearInterval(myInterval);
trace("clear interval");

[dawsonk]

on (release) {
	clearInterval(myInterval);
	var myInterval = setInterval(slow, 33);
	var counter = 0;
	function slow() {
		if (counter<20) {
			counter++;
			trace(counter);
			trace("slow function");
			prevFrame();
		} else {
			clearInterval(myInterval);
			trace("clear interval");
		}
	}
}

[@]

/nod

[M4X]

Beautiful, thanks so much.