Have you checked your throttle body and see if it needs to be cleaned or not? Could also be a dirty MAF sensor in the airbox. Normlly, when you kick the A/C on, the ECM will respond to the increased load by increasing idle speed. This is done via the IAC valve or Idle-Air Control valve - which is part of the throttle body. If the throttle body is excessively dirty (carbon deposits, clogged or stuck PCV valve, varnish deposits from gasoline, etc.) - there is a good chance that the IAC valve is also clogged up. If it is sticking or clogged, the idle will be erratic and uneven when the ECM tries to bump up the idle speed.
