Fast rotate

I hope somebody can help me because I’m stuck with my flash and I need to finish it soon.
My problem is that I have a movieclip with an object thats rotates non stop :sen:(animated from 36 frames). Now I’m trying, with one-clicking a button, that the moviclip will double the speed/accelerate and stop at a frame 20. But when the playhead is 10 frames in front of frame 20, the playhead will ignore the “stop” but will do stop when it arrive for the second time on frame 20. (I have 6 stop buttons (stops at frame 10,15,20,22,30,35))

This is what I have sofar (AS2.0):

var intervalID:Number;
button.onPress = function() {
    intervalID = setInterval(moveFast, 10);
function moveFast() {
    if (mc._currentframe==140) {