From a cold start during mornings the RPM level is usually higher than normal, this is due to your automatic chock at work. This is to compensate for the cold engine which is usually "nanginginig" . There is a mechanical lever/vacuum connected to the temperature sensor and throttle (carb). When a certain temperature (operating temperature) is attain the lever pushes or pull, depends on the car make. You will feel this sa gas pedal mo. You will notice too that the RPM goes down significantly as you reach the operating temp of your car.

Oh btw, I forgot to tell you to check your ignition coil. If you say that your RPM goes down when your car gets hot then this is one characteristic of an hot coil that's going south.

Try spraying the carb and it's linkages with WD40. You might have a sticking linkages.

Observe your car for a day without aircon. Then another with it on. Keep us posted.