Stalling problem

Hi
Hoping someone can solve this?
When i have air conditioning turned off the engine stalls no end of times and can be quite disconcerting as the steering is really heavy. Connected a laptop to diagnose but no problems shown up for the engine or gearbox, had taken the A8 for a gearbox service but they refused saying i should sort the stalling out first. As soon as i switch air con on the fault disappears. I have been told that when you switch air con off the revs drop 2 to 300 on tick over. Surely this cannot be correct and must be some kind of fault.