Hi.
I have a 2009 A6 Avant 2.0tdi CVT with 70,000 on the clock. When I first brought my car 2 years ago (48,000 on the clock) I noticed a slightly similar issue.
(Only ever in winter) the engine was very sluggish for the first 5 minutes of driving and the power seemed to be all or nothing... It also did as you described with the revs jumping around at idle. This all went away after driving it around a bit.
I took the car to Audi as it was under warranty and they came back and said that this issue was completely normal and a characteristic of most of Audi's 2.0tdi engines in cold weather.
Phone Audi UK and ask for technical advice, they are very very good for customer service and will help you all they can. Tell them your problem and ask them to investigate as to weather it is a common thing on your engine or not.
Does the car seem to be lacking power and any certain revs
If you are sure it's a misfire or any kind of fault your car will flash up a problem with a light on the dash. Cheapest thing to do is buy a fault code reader (no need to spend any more than £30) and run a diagnostic yourself. A misfire will 100% definitely show up during that.
Hope this helps!