insert into your document class

var frameManager = new frameRateKeeper(24,this);
addChild(frameManager);
// where 24 is the frame rate you want, then insert the following code into a file called frameKeeper.as
package{
   import flash.utils.Timer;
   import flash.display.MovieClip;
   import flash.display.Stage;
   public class frameRateKeeper extends MovieClip{
      var requestedFrameRate = 30;
      var frameTracker = 0;
      var currentFrameRate = 30;
      var stageInstance;
      var secondTimer:Timer = new Timer(1000);
      public function frameRateKeeper(num,sI)
      {
         stageInstance = sI;
         requestedFrameRate = num;
         currentFrameRate = num;
         this.addEventListener("enterFrame",function(){ frameTracker += 1;});
         secondTimer.addEventListener("timer", adjustFrameRate);
         secondTimer.start();
      }
      public function adjustFrameRate(ext)
      {
         var curRate = frameTracker;
         if(curRate > requestedFrameRate)
            {
               currentFrameRate --;
               stage.frameRate = currentFrameRate;
            }
         if((curRate < requestedFrameRate) && (currentFrameRate < 90))
         {
            currentFrameRate += requestedFrameRate - curRate;
            stage.frameRate = currentFrameRate;
         }
         //trace("stage set: " + currentFrameRate);
         //trace("actual: " + curRate);
         frameTracker = 0;
         //stageInstance.frameRateCounter.text = stage.frameRate;
      }
   }
}