Sounds like the limit switches inside the motor itself have got their knickers in a twist. Theoretically, it is possible to strip the motor dow, clean all of the old gunk out and regrease, but I have to confess I've never had any long term success doing that.
Cough up for a new motor, or better still a decent second hand one from Ebay.
Not sure how easy it is to change on a 1998 Passat, but on a 1995 model it's a sod!