Alight, I'll bite. Here are the things that have caused a rough idle for me in the past (not just on a volvo, but on all kinds of cars):

Bad temp sensor--this was on a our v90. Car ran like total crap until it warmed up. Thing was, you could see the temp guage shut down whenever the sensor acted up. AND it threw a code.

Cheap gas--My V90 idles and runs like poo on the 87 octane. I can always tell when its been refilled on the cheap stuff. AND if you run the cheap stuff, it sets off the knock sensors and your timing gets all retarded (literally) on you.

Knock sensors. As I've learned this weekend, you've got two of these things hidden deep in your engine bay. A faulty one of these things will leave your car running like crap.

IAC Valve-- When was the last time you actually cleaned this thing? They get really gummy on Volvos.

Intake Hose--Look for leaks in the air intake hose behind the MAF. IF your hose isn't sealed up tight after the MAF, its readings are all wacko, and you're getting the wrong fuel/air mixture.

MAF-an MAF that's off calibration (old or in need of cleaning, which I don't know how to do) means you car just will not run right.

Vacuum leaks--I hope I don't have to explain this one to you. Luckily there aren't THAT many hoses on this car, so it should be pretty easy to track down. If your engine is leaking oil from an undisclosed location behind the alternator, then you might have the dreaded (and very common) Oil trap vacuum leak. You can read mine and Kenner's posts about that nightmare on this board.

Bad EGR valve--Never had to deal with one on a Volvo, but it sure made some of my other cars run like crap. EGRs only operate to about 2500 RPM, so if the car's happy at higher speeds, you may have found your culprit.

Shitty air/fuel filters-- please tell me you keep up with routine maintenance and this is a non-issue.

Anyway that's all I can think of for now. Hope it gives you a place to get started.