I have a '99 s80 that also has the same problem. I took the car to the shop today after driving the car for about 20,000 miles without getting it checked out. several times throughout the year, the get service urgent warning would come on, the check engine would come on, and the emmissions system failure message would come on, usually all around the same time! My car idles terrible. The code came up P0121 and the mechanic said it needed the throttle position sensor replaced but there are no replacement parts to be found. They said the dealer will also not be able to find the part because it is simply not available. By the way, my car has not idles correctly since it was new...I talked with the regional Volvo service guy and he said it was "normal".

Maybe that is why your car is still having the problem...they didn't fix the right thing? They also had a code P1603 and had no information for this code.