If you are suffering from an unstable idle, the fix is to clean the throttle plate. Oil and gunk can build up on it, causing the problem. As usual, the fix is much easier than getting at the parts you need to clean.

Start by removing the engine cover, the intake hose, and the 3 10mm bolts that hold the vacuum switch plate to the bracket. Unplug the harness connector on the intake sensor on the throttle boot.

 

The next step is actually much harder than it sounds. The first thing to do is to unclip the breather from the throttle body boot. This is MUCH harder than it needs to be. There is next to no room to get your hands in there to squeeze the clip to release it. I ended up using a long prybar to work the side away from the firewall, and my hand on the side closer to the firewall, and then pulling it off.. Naturally, because it's so brittle, it's likely to break at some point.

Once you have the breather off, remove the Allen bolt. On A6 cars, this is not too hard, there is a lot of room between the firewall and the engine. On A4 cars, it's a giant PITA. Once you get the Allen bolt out, the TB boot is free to slide off, towards the firewall. Now the fun begins.
See the gooey icky crud there? That's what gets all over the throttle. It comes from the crank and valve cover breathers. This is all part of the car's way of protecting the environment. Spray your cleaner here, clean this out well.
I didn't take any pics of the throttle plate, as it's nearly impossible to see without a mirror, so photographing it would be pretty tricky. But, if you have a mechanic's mirror, you can stick it down there, and shine a light on it, to light up the inside of the throttle opening. You want to take your carb/choke cleaner of choice ( I like Berryman's) and spray it in the throttle opening as best you can. Take some paper towel, and work the gunk loose. Take some clean paper towel and spray the cleaner on it, and wipe the throttle, you should be getting lots of oily gunk out of the throttle. Keep looking at it with your mirror to see how you are doing. You might also want to open the throttle up a little, and carefully wipe the edges of the butterfly plate. Let it all dry out, and put it back together.