Yaris 2008 1,3 , with arco on, low rpm stationair

[Nono]

Hello, I am new on this Forum. My wife has a Yaris from 2008. We bougt this car in 2009. Totall km is now 117.000 km. No real issues yet. Only leaking shock adsorber behint and both rear weel bearings. Since this week, when the airco is on, the rpm becomes so low that the motor is shaking a little. Not a real issue, but I would like to know what could be the cause. I am sure that the motor runs on 4 cylinders. But shakes due to a low rpm, about 500.without arco on the rpm rises 50 and problem is gone. I did myself the regular maintenance, oil change, sparks,, brakes etc. The car runs ok.