I had this idle problem, was charged $600 w/o any results
and was told it'd take another $800 (VW repair sucks)

I tracked it down to the idle temperature sensor.
Seems that it is a thermsistor that senses when the
engine is cold. I added a 500 ohm resistor in series
to it. Seems to have stopped the problem, although
the idle is a bit low (800 rpms).
(500 ohms is about the middle of the temp chart
in the repair manual) (this is from memory, but
i'm pretty sure about the actual value)

This sensor is located on the drivers side, towards
the rear of the van.

Ron