If the fan is stop starting with the aircon on then most likely the low and medium speeds have failed
The Trinary switch in the aircon is then running it at high speed in response to an increase in refrigerant pressure once pressure drops fan stops this stop start behaviour will because of the current drawn also cause a momentary dip in the idle speed.
If you fancy the 70 Mile drive over here at the weekend I can put your car on Testbook and test it but if you have a local dealer they can do the same and it shouldn't cost you more than half an hours labour.
What year and mileage is your car?
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