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 duct.
The next step is actually much harder than it sounds. The first thing to do is to unclip the breather from the throttle body duct. 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 duct is free to slide off, towards the firewall. Now the fun begins.