Well in that case, we need to know exactly "what" was done in that service. The reason for my query is the service procedures tend to vary from mechanic to mechanic; at least that's what prevails in our country.
Here is a little thing about whats going on inside the engine bay of your car:
The AC system is faulty, has some sort of hindrance in smooth operation. It does work, but puts extra stress on the engine. Its a factory installed function in the car, that whenever engine RPMs fall below a certain threshold, there is a relay which shuts off the AC system automatically. It resumes normal operation as soon as engine becomes normal again.
You must have heard a little "tick" sound from the engine bay whenever you turn AC on or whenever you attempt passing a speed breaker slowly in the 2nd gear, as soon as the engine revs drop, AC shuts off with a tick.
Now in your case, AC puts stress on the engine due to some cause, engine drops revs in idling and the relay shuts off the AC. Thats why hot air blows in the cabin but resumes normal operation if you revv it up.
If you look closely at night time, whenever this happens, you'll notice sparks near the compressor pulley when AC turns on. Basically there is a little jam in your system or the AC clutches are gone.