I had a 16V, and a few friends who did too...

We ALL had some sort of idle problem very similar (or nearly identical) to what you describe. The idle control valve is often the culprit. Try cleaning as was previously described. You may need to replace it ultimately though.

I also had my fuel distributor cleaned as well and alot of crud was found inside.

Over the years I owned it, I don't know that it EVER really idled "perfectly" for me...

J.G.