I think Mark's suggestion about the ground wire will cure the problem:

"Another possibility is the ground
wire for the OX sensor. This ground wire is not in the same harness as
the other OX wires. It is the lone brown wire in the small bundle that
goes to the distributor connector but split off to a ring connector that
is then screwed down to metal on the engine. Check that this ground wire
is making a good contact to the engine."

The ground terminal for this wire is most likely located on the left
engine top, below the air conditioner compressor. I have also seen them
attached to one of the oil breather mount bolts - not the greatest place
for a ground, considering the breather tower is plastic.

Clean this ground and get a good contact with metal and this "running
rich" problem will likely be fixed.

Michael
My VW web page:
http://www.velocity.net/~vickrail/moshevw.htm