So after a couple hours part-swapping and generally diagnosing, I've
come up with the following:
Fuel delivery: OK
Timing: OK
ECU: OK
Air flow meter: OK
Cat: OK

Driving up and down the street clearly shows when the problem shows
up. When the idle switch is engaged the engine runs and idles just
fine. The second the idle switch is disengaged, wheezing, coughing,
and associated engine melodramatics. So what does the ECU ignore at
idle that might cause? Too tired to continue today. Tomorrow the
multimeter comes out and the wiring harness and sensors get tested.

--
John Bange
'90 Vanagon - "Lastwagen"
'90 Vanagon GL - "Wiesel"