Does this happen when coming to a stop as well as idling? I had a
problem on my old Polo where the revs would drop and even stall when slowing down. Turned out to be a
problem with the ECU software - apparently very easy to fix. Ran like a dream after this was diagnosed. Maybe something to consider when you take it back.
Also, might be worth taking your car to another garage, that has diagnostic tools. Sometimes, a new mechanic/garage can detect a
problem very quickly.