Changing setInterval at different times

I am quite new to actionscripting - using flash 8

I am making a flash game. The object of the flash game is to use the mouse and dodge the beer bubbles.

to start with, the beer bubbles are released slowly, after 10 seconds i want the interval to decrease and the amount of beer bubbles to increase. my code is as follows:

if (cTime <= 10000){
setInterval (makeNewClipB, 1000);

if (cTime >= 10000){
setInterval (makeNewClipB, 500);

However, regardless of what the timer is at the bubbles are released at 500ms-

not sure if the method i am generate the bubbles has anything to do with it:

function makeNewClipB() {
ran = (Math.random()*2000)+1000;
ranID = setInterval(makeNewClipB, 1000);
newClip = _root.attachMovie(‘bubble’, ‘bubble’+depth, depth++);
newClip._x = Math.random()*Stage.width;
newClip._y = 400;
newClip.speed = (Math.random()*10)+5;
newClip.onEnterFrame = function() {
this._y -= this.speed;

If someone can oint me in the right direction to get this working i would be very appreciative - i am starting to go mental :h:

my .fla is here

Many Thanks