OK,It's not real easy to diagnose a car without having it in front of me but here are some ideas.
You said it "jumps" from 600-200rpm. Since your car is a euro version, I'm assuming that the battery is in the engine compartment and NOT in the trunk (like on USA cars)-First thing to check, Look at your battery posts.Do you have a "small wire" that is connected on the pos side post. Make sure that you don't have a problem with corrossion or breakage on the little "wire" that is connected to the pos batt cable at the post. If this wire has a bad connection, it can play havoc with the idle circuit. I have seen this on US version 126 chassis cars.
Once you have made sure that is OK, you need to be certain that the ign sys is in good working order. (the rough running you described sounds like a miss to me, so you may have more than one problem)
Have you "ohmed" the plug wires- to make sure you don't have a plug terminal or two messing with you?
Have you checked the plugs, to see "how they look" as far as how you engine is burning?
Is the dist cap & rotor ok?
I know these seem like stupid things to ask, but alot of times an itermittent problem that seems complex is actually a simple thing.
When the engine is running "warm" have you pulled the connector off the idle air valve to see if it "speeds up" and then settles back down when you re-connect?
There is a procedure for checking the idle air valve and control unit, with a volt meter ,I think (going from memory here). I can look up the test tommorrow, but it should be on your CD.
On your "added questions", I think you are correct, but don't have any of the CD info or books here at home.