First off, gorgeous car.
You're going to want to try the A8 & S8 D4 forum since your A8 is already in the new generation. Also you might want to look around other forums since this one is not that active if you want a solid answer. Don't even know which direction to point you since I haven't really come across a really active D4 forum yet. You might want to try audiworld?
As for the slippage you're still under warranty, just go right over to the dealership and have them go over it with a fine tooth comb. Try to fix the problem before it gets worse or causes more trouble.
Good luck buddy