John,  I have cruise control on my Turbo Diesel  and it appears to react in
the same way as yours.   It is a quite simple mechanical device, just a
throttle lock and quick release mechanism.    IMHO the symptoms  you  list
are generally normal for simple fixed setting throttle cruise control.
(engine has fixed fuel imput = fixed road speed) except when load exceeds
fuel requirement (hills/ long grades) when loss of road speed occurs.   I
find it quite usual to have to just put the weight of my foot on the gas
just to maintain road speed on long grades or hills,
This is due to mechanical system limits,  to have a system that applies more
fuel or retard the exhaust to slow the vehicle to maintain a fixed road
speed (+ /- 5%) require complicated sensing and control and fuel management
systems.      (if yours has a vacuum servo to control the throttle setting
check  it for air leaks or may be weak/broken springs on throttle linkage)

"Dusty and the Luggage"
-----Original Message-----
From: John P. Stoneback <jps@CATWOMAN.CS.MORAVIAN.EDU>
To: Vanagon@VANAGON.COM <Vanagon@VANAGON.COM>
Date: Friday, May 29, 1998 5:34 PM
Subject: Cruise Control

>I swear there is stuff in the archives on this but I couldn't turn up
anything
>on a quick search so here are my symptoms.
>
>Until my last trip, setting the cruise control always resulted in the speed
>being set to just a hair above the present speed. Operation was thereafter
>relatively stable.
>
>On my last trip, setting the cruise control freqently resulted in the speed
>dropping 3-5mph. Then it would seem to recover but often allowed the
vehicle
>to slow down up to 10mph at intermittent points. Sometimes this seemed to
>coincide with hills but not always. Clearly something is amiss.
>
>Bently gives procedures for checking failures but this is not a complete
>failure. Does anyone have suggestions of what to look at or how to
diagnose?
>
>John  ('89 westy)
>