Found this (& lots more) at:  http://import-car.com/magnav.htm
go to 'Import Car' at left, then to 'search back issues', type in
volkswagen or whatever strikes your fancy, 'all issues'.

Austin  (ps - I just highlighted/pasted the following directly off the web
page...it looks OK as I'm getting ready to send it (Eudora), I *hope*
there's no weird formatting that gets ugly on the receiving end-I'll know
soon enough!)

Heated Oxygen Sensor Precaution

When the heated oxygen sensor (HO2S) operates too slow or is out of range,
it causes hesitation when cold and poor idle quality. This condition could
result from the application of contact cleaner or other similar substances
to the sensor connector.

As the sensor heats up, air inside the sensor expands and vents out and
around the tiny spaces surrounding the wire strands within the wiring
insulation.

As the sensor cools, there is a reverse effect which causes minute suction
in the same area around the wires.

If contact cleaner has been sprayed onto the terminal end of the wire, it
is possible for the internal suction to draw these materials down through the
insulation of the wire and into the interior of the heated oxygen sensor.
These materials can then coat the sensor surface with contact cleaner and
destroy or alter proper sensor operation.

Do not use contact cleaner or any similar material when repairing or
replacing heated oxygen sensors because they could destroy proper sensor
operation. Replace the sensor if you find corrosion on the sensor terminals.