I just corrected this same problem on my 91 Syncro Camper.

The ground wire for terminal 19 on the ECU was the culprit.

On non-Syncro Vanagons, the ground point for this wire is on the left
engine head, under the intakes. On my '91 Syncro, it is on one of the
bolts that holds the oil breather tower to the top of the engine.

I cleaned the ground point and the connector, AND added a new separate
ground wire between the oil breather bolt and the lower clip attached to
the distributor.

Three days of troubleshooting was solved with this simple "fix"'.

I would try this first before you replace any parts.

Good Luck

Michael
87 Syncro GL
91 Syncro Camper
93 Eurovan MV 5-speed
95 Eurovan Camper
99 Eurovan GL
99 Winnebago/VW Rialta

My personal VW webpage:
http://www.velocity.net/~vickrail/moshevw.htm