Engine sometimes goes off while stopping at idle

Hello.

I have a problem with my A8 3.7 quattro with Tiptronic.
Sometimes it happens that when the Tiptronic is uncoupled, the engine does not go to the idle speed, but goes far below it (about 300 rpm) and the red generator lamp already lights up. The engine usually catches and the rev indicator goes back to 750 rpm, but sometimes it also fluctuates between 600 and 1000 rpm and always commutes back and forth.
This problem only occurs when the engine is warm. I have the feeling that the engine is under idle speed, especially when uncoupled from the 5 speed (motorway exit). But it also happened when stopping from about 50 km/h.
Unfortunately, there is no power steering without a motor (Bremse still works). So at about 70 km/h on the motorway exit the engine went out. I had to crank up a lot to get the turn of the exit without a servo (very dangerous). The Tiptronic also doesn't like it when the engine is restarted with the starter while driving on N. When switching to D, it then makes a big set, because the speeds (motor/gear) do not fit.

Has anyone of you had this problem?

Audi is unfortunately at a loss, as the error is rare (always when you need a running engine, like the motorway exit) and otherwise there are no error messages.

Thank you for every contribution.

Martin