I have an 87 Westy that also had hesitation. Tried everything, replacing the Hall unit, the distributor, clean the gas tank, etc ; even had the factory in Hannover 
Germany look at it while I was living in Denmark; NO LUCK in fixing the problem, 
UNTIL I by accident found the following VW Technical Bulletin.

Here is a copy and it applies to all watercooled Vanagons, not just the Digifant ones (there is an updated bulletin no.92-02 that states all waterboxers) ;

Technical Bulletin
Group:     24
Number:  92-01
Date:       Feb. 29, 1992
Subject:    Vanagon Intermittent Misfire
Model(s)  All Vanagon Digifant Fuel Injection Engines

CONDITION
After driving for � to 2 hours at a constant speed, vehicle experiences lack of power, cuts-out or stalls, often accompanied by bucking or jerking of the vehicle.
In ALL cases, the vehicle will restart immediately. This symptom may not re-occur for several weeks/months. Inspection by dealer results in nothing conclusive.

Some air-flow meters may experience a vibration resonance of the meter potentiometer wiper during extended constant driving. This resonance causes the air-flow 
meter to supply an intermittent signal to the Digifant ECU. The ECU will then default to a "no-load" condition and reduce the injection time.

SERVICE
Harness, Part No. 025 906 302, is available to resolve this condition.
* remove electrical connector from air flow sensor
* install convertor assembly, Part No. 025 906 302, between harness and air flow sensor

NOTE
If customer complaints persist after performing the above repair, contact your Zone Product Support Specialist  (TSM in Canada)
*************************************************************************************CAUTION
* Part numbers are for reference only. Always check with your Parts Department for latest information.
************************************************************************************
"

Thats what the Tech Bulletin says. I went to a local VW dealer,  Niello in Sacramento, CA.  They had this part on the computer and ordered it for me.
Harness costs about $100.00 US, very simple to install. Takes about 3 min. max. The new harness fits between the old harness and the silver air-flow meter box 
attached to the air filter box. Remove the silver clip, wiggle out the big wiring connector (from the side of the air flow meter), connect the proper end of the new 
harness to the big wiring connector you just wiggled out, put the new clip into the new connction, then take the other end of the new harness and plug into the air 
flow meter connection. Put the silver clip back, and presto you are done. I did this and have driven jerk free (aside from those behind the wheel on the road) ever 
since.

I hope this is of help.

Hans Peter

87 Westy
71 Sandrail