I'd start with cleaning the throttle body and earth points.
On mine I had fluctuating idle and after replacing the coolant temp sensor (came up on vagcom) it turned out to be the gearbox earth was loose
Also check the switch on the throttle body. Yesterday just before I put my car on the dyno it started fluctuating again, and today I had a look around and noticed the switch plate was loose! So I repositioned it and tightened it up, seems fine now
Remove the ISV (under the plastic where the ignition leads are held on to) and give it a soak and clean with carb cleaner. You can connect a 12v battery directly to it to make it spin so it washes it out (don't jump when you first connect it up, because it's very noisy and it jolts!). Once you're happy with it, spray some WD40 inside to lubricate it (carb cleaner is very dry) and refit, see how it goes. Make sure your breather pipes aren't split or disconnected.
If it still idles bad, replace the ECU relay, go to a VW dealership for that, it only costs about £16 last time I checked).
If again it still idles bad then I'd look at trying a new throttle body (the switch on it may be faulty) or a new coolant temp sensor, again go to VW for it, it cost me £20 I think it was.
I suggest plugging it into vagcom too
