Installing a vapor barrier is fairly simple but you need to make sure you are doing it correctly or you'll be wasting your time. This website has a complete section on vapor barriers and how to install them:
http://www.diyhomeinsulation.com
I had the same problem. It can be anything from dirty filters to a bad thermostat. This website has a section devoted to air conditioners and how to troubleshoot them. It helped me with my situation.
http://www.diytoday.net
