I've had this problem with 2 of my VWs in the past. More than likely
it is as simple as on my cars. More than likely one of the butterflys
on the Throttle body is not closing all the way. Next time you drive
around and it's idling at 2000rpm pop the hood and just push a little
on linkages towards the closed postion. More than likely the RPM will
drop if you push it closed a little. You can't tell just by looking.
If that's the problem try cleaning them up with some Carb Cleaner. If
that don't work just get a new T-Body off of another car.