Ji brother i had this problem aswell, when you turn Ac on, RPM drops and car starts vibration or rattling, because Engine Computer is not connected with any ELD (electronic load distributor).
so i dont know about Cupis, what that is....
but solution for this issue is to get it done thru some expert electrician, who will get wire from ECU, there is AC wire, that tell ecu if AC is on or of, so he connect that wire to ELD and when AC is turned on your RPM will increase, this is the proper solution, other wise mechanics have many juggarrs for this issue, like Vaccum installation on your Accelerator wire, choke system when Ac is turned on so rpm will increase.