The story so far:  about two months ago a sudden increase (or should it be a
decrease?) in mileage from 27 mpg to 10 mpg. About a month ago, increase of
about
ten thousand percent in black smoke emission.  (Sure discourages tail-gaters!) 
Lots of excellent advice and suggestions from you folk, change of ECU and Temp.
sensor 2, adjustment of AFM.  Minimal improvement.

Friday, finally took it to a mechanic.  It took him three hours to figure out
what was causing the problem and I think it was a great piece of detective work.

Wire coming from the O2 sensor on the 84 is a single lead that plugs into
another
lead coming out of the wiring harness.  That harness lead goes into the multipin
connector.  For reasons known only to themselves, VW engineers made that harness
lead a co-axial cable with the shield grounded.  Somewhere along that lead,
inside the harness, the insulation around the inner wire broke and allowed the
signal to ground on the shield.  

The mechanic, Niel, took endless trouble to explain the operation to me.  Seems
at 600 degrees the O2 sensor generates a 0.5 volt signal to the computer.  Over
that voltage indicates a rich mixture and the ECU decreases the fuel flow.
Under
0.5 volts indicates a weak mixture and the ECU feeds in more gas.  A grounded
wire will produce a 0.0 voltage so guess what the ECU does?!!.  Niel simply took
apart the multipin connector and checked the voltage at the pin, then cut
off the
faulty lead and replaced it with a single wire lead.  Now ole Smoky purrs like
a kitten and I am looking forward to 27mpg. again.