I've been wrestling with this issue for a couple years. It's gotten more
frequent, now.

When idling, the engine (1.9L) rpms hunt up and down.

Sometimes.

It will keep doing it until I put the van into gear. Drive up to the
bank's drive up window, put it into park, and it will start doing it
again. Sit at a traffic light with foot on brake and transmission in
Drive (AT) and no hunting at all.

I asked a local VW mech about it, and he bypassed the idle stabilizer
electronic thingy (plugged the two cables into each other) to see if
that was the issue. For a few months, no problem. But the issue has
re-occurred with the stabilizer thingy still bypassed.

I went over the air intake system quite closely three years ago, and
found no leaks.

It's the "it stabilizes when transmission is put into gear" thing that
seems a bit of a clue.

--
Rocky J Squirrel
'84 Westfalia: Mellow Yellow ("The Electrical Banana")
'74 Westrailia: (Ladybug Trailer company, San Juan Capistrano, Calif.)
Bend, OR
KG6RCR