John,
I had an '89 Jetta that had similar symptoms.  The cure for that car was a bottle of Autobaun Injector Cleaner  every few months.  (this fix was recommended by the VW parts counter guy when I went in to price an air flow meter that was recommended by the mechanic at a local garage)  The problem was gone after 1/2 tank of fuel.  I am sure other fuel system cleaners would work as well.