Hi all wondering if anyone has any thoughts on the following issue.

Brought an A3 2.0TDI 2008 manual gearbox from an audi dealership about 3 weeks ago. The car has an intermittent issue which Audi are having trouble fixing and its starting to get frustrating.

When I stop at traffic lights for example and put the car in neutral then take my foot off the clutch the revs will sometimes drop from 800-820rpm down to almost 600rpm, at which point the car feels like its going to stall, but then after a few seconds will pick back up to 800rpm.
It never occurs if i keep the clutch pedal down however.
Its occurs more frequently when the car is cool but still happens sometimes when it is at running temperature.
It also can feel hesitant in 3rd gear when accelerating.

Audi have tried replacing the fuel filter, cleaning the EGR valve and the throttle body but the issue still occurs. Each time they have found no fault codes.

Any ideas?

Many thanks for reading

Adam