Flash with high cpu usage

my flash has lots of images and dispaly objects. Hence it costs high cpu usages. after using my flash for an hour, it is dieing.I am trying to reduce it. I have some questions here:

  1. I create an array, or string at frame 1 for example, when I go back to frame 1, should I create a new array, or a new string?? Or is it better to make it empty array(eg:[]) or empty string("")? which way it reduces cup uasge more?
  2. does anyone know how to force the flash to do garbage collection??
  3. any other possible reasons that cause high cpu usage? any idea?