when i turn my defroster on, while i am idle, my car will rev itself from 500rpms to 1000rpms. it does so at equal intervals (evry few seconds). any reasons for this?

 

It the AC. When you hit the defroster the air conditioning circut is enguaged and the compresser cycles on and off this raises the idle speed to compensate for the extra drag of the compresser.
Question, why is you idle set @500rpm? Its kinda low even for a standard transmission.