I would bet that your problem lies here:

Look for a ground wire (brown) that comes out of the sheath of wires that
goes to the base of the distributor (Hall Sender). The brown wire I am
mentioning is a ground wire for the ECU (terminal 19, I think).  This
ground wire terminates either under the left-side engine intake, or (more
likely) on the oil breather tower.

If this wire terminates on the oil breather tower, move it to a bare-metal
location, or add a second ground wire to it that does. In the event you
find the wire terminates at bare metal already, clean it up.

In my experience as an owner of over 30 water-cooled vanagons, I am fairly
certain that having this ground making a good contact will solve your
problem.

Michael Vickey

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