How do the plugs look?
You want to properly test stuff, likely culprits being vac leaks, MAF problem, IAV problem, CLT sensor problem, fuel pressure problem, faulty injector. Sounds like only misfire at idle and not at raised speed?
Yeah idle is where it's missing which is annoying, if it were throughout the rev range then I'd say it'd be a lead/plug/rotor etc. issue and much easier to sort out really.
I have had MAF issues before so it's a potential culprit and I do have a spare I can fit and tune (a year or so ago I tuned the car to work spot on with this MAF).
Idle control valve (ISV in mk2 Golf terms) is the cylinder above the rocker cover that I unplug and it dies so chances are it isn't that, although they are known to be a awkward...plus they are £100+ new and second hand is unknown if they work.
Blue plug I undo is the coolant sensor for the ECU and again, unplug it and it dies. I've also done some resistance readings when cold and when hot but I don't know what the correct values should be. A mate has a spare so I'll get that and chuck it in to see if it makes any difference.
I will do some proper diagnosis soon, but wanted to check today if the ECU changed fuelling etc. when I removed plugs, if nothing changed then I'd have an obvious sensor issue...but all of them caused the engine to change. The fact it's fairly random and only really at cold idle does point towards the ISV playing up as it has randomly increased the idle recently with a blip of throttle then getting it to settle down.
@InvaderGIR. Mine had a really rough idle due to the breathers being blocked that connect to the flying saucer thing on top of the cam cover, also clean the isv out. Probably loads of gunk in those hoses.
It would often stall when engine is cold
I cleaned the ISV a while back when it was doing it's random idle increase but it wasn't very dirty anyway haven't touched it in some time though, I'll give the saucer a check too.
The issue has only really come up in the last 6 weeks or so, before that it had a slightly dodgy idle for a couple months (which might well be the ISV dying to be fair).
It's always sounded like a diesel tractor so I'm not fussed about that part of it.