That weird oscillation explained

[David Head]

If the ISV fails in a 'mid position', it will flow too much air and keep
the idle/sensor plate up. However, manual Audis have the old throttle
switch and decel valve. The ECU sees the throttle switch go closed and
sez - woo - open that decel valve to get that sensor plate down! At 1200
rpm the ECU sez - that's far enough and orders the decel valve closed -
that allows the sensor plate to slowly go to the rest position. As soon
as the decel vavle goes closed - since the ISV is flowing too much air,
the sensor plate rises back up. The cycle repeats itself every second or
two - very embarrasing sitting at a light...

Occasionally you can clear it by unplugging the ISV while running, shut
off engine, plug back in and restart. If this or a thorough cleaning
don't solve it, time to go to the junkyard for another... Too darned
expensive new.