For the last couple of years I've been fighting a cruise control that just won't hold a speed. I lubed throttle linkages and checked vacume but no help.
Over the weekend I had pulled the instrument housing to repair a stuck odometer and to lube a speedo cable due to a vibrating speedo needle.
Well, now the cruise control maintains a set speed, very smooth transitions. I've had the instrument housing out over the last couple of years so I'll eliminate electrical contacts being reset at the speed sensor pickup at the back of the speedo. The only thing that makes sense to me is that if the speedo needle was jumpy becuase of a sticky speedo cable then the sensor was not able to "listen" to a stable speed.
Hope this helps someone.
1980 450 SEL
1986 280 GE