Jim, I have cruise control on my 88 and several years ago it started to fail
intermittently.  I traced the problem to a pin hole in the actuator
bellows.  It's the part of the system that sits there on top of the hot
engine and has to do all the moving.  I verified the bellow were bad by
trying to pull a suction on it and could see that it wouldn't hold a
vacuum.  I temporarily patched it with a little dab of silicone sealer on
the weak spot (which I let cure overnight).  Replacement followed as soon as
the part was ordered from BD and that actuator is used on multiple models so
it's available.

Don't know if this is your problem, but it's a likely failure point for a
moving part that's 16 years old and easy to check.

Good luck.

John Graham
88 Westy
Charlotte NC