When your air con is on the econ setting, the compressor does not run. When you select the air con to operate, the compressor starts up. This will put a load on the engine, demanding power so the engine wil speed up slightly. Leave the car idling and put on a heavy power accessory such as the heated rear window or the headlights and the same thing will happen.
When any engine is idling, it is only developing enough power to overcome internal friction and to drive the anciliaries (alternator, power steering pump, etc). If you add any other demands to it, it will alter speed accordingly.
If you operate the air con on the move, the enigine is already at relatively hgh revs and power output is higher, so you simply wouldn't notice any change. I don't believe you have a problem.