With idle issues for any motor, you need to look at a few things. First, a good visual once over can solve a lot of problems, but it sounds like you have that covered.Several thing syou will need to measure: manifold vacuum, fuel pressure (with key on engine off, and with engine on), and if you have a good tach, actual rpm range your are dealing with. What is the temperature doing, and is the thermostat working correctly? If the engine never passes a temperature point set in the computer, it will always operate in open loop. If you have access to the right type of scan tool, you can read a lot of this information (it is surprising that whoever did your tuneup didn't scan the engine?). Look for fluctuations in vacuum or fuel pressure. Both will give you hints at the problem. I would also pull the new plugs and verify that they are a suitable heat range, and gapped correctly. I would also verify that they are all working correctly. If fuel pressure is in the acceptable range for your vehicle, and manifold vacuum is fluctuating or low, I would look for either a vacuum leak, injector leak, hunting IAC, or possible bad/ misadjusted TPS. Those would be the first targets for me. Many sensors could be involved, and it is way to expensive to just replace everything. Afaulty EGR system could give you similiar results if the valve was stuck open/closed or pulsating.If fuel pressure is low, it is either the pump, regulator, bad injector or a fuel leak. I believe your motor is turbo charged, and this can also add problems. At idle, the turbo should not be making boost, or very minimal. If you are getting positive readings on the manifold vacuum at idle, you may have an issue with the turbo, though that is out of my realm of experience. Another concern, particularly after someone performs a tune up is the timing. If your vehicle has a timing belt (which I am sure it does), it is a wear item, and should be replced on a regular schedule, if this was part of your last tune-up, it could be miss installed. This is a bit more on the unlikely scale, but I have seen it happen, and a few degrees advenced or retarded will generally not make a huge difference when the engine is at speed, but can make a big difference with a computer controlled vehicle at idle, as the camshaft position is no longer known accurately by the computer, this can cause the engine to advance and retard as the computer tries to balance out the sensor information with its expectations. Unlikely, but I would check it. Does the vehicle display these sysmptoms when cold started? If your sysmptoms show up in open loop and closed loop operation, that narrows down what components and systems can be involved. Obviously, the O2 sensor information cannot be blamed if the vehicle exhibits these problems in open loop. I am sorry I connot give you specific information for your engine, I have never worked on one. The above thoughts apply to modern motors in general. I don't know if your engine uses a MAP, MAT, or MAF, but either which way, if the computer is not getting correct information in regards to the airflow into the engine, this could obviously cause similiar prolems. I am assuming you inspected the Air intake tract for dirt and debris? If you make a big round sensor that the air flows through between the air cleaner and the throttle body (or turbo in your case), there is usually a fine mesh screen there that protects the element. If so, very carefully clean this screen. Last but not least, I would definatley pick up a Haynes or Chiltons manual for your car (or better yet the factory manuals though they tend to be a bit pricey and hard to use). Both Haynes and Chilton are pretty good and showing you how to test the majority of your sensors and actuators. Can save you a bundle. I know the Haynes #96023 covers the A4 from 96 - 01, but I do not know about the 2002. Even if it doesn't cover your specific motor, it can give some great info on testing the sensors. Good luck.