The cruise control in my 84 300td is a bit jumpy, especially down slight hills. I don't usually use cruise at less than 65mph, but it's better the faster I go.

Some things to note.. It used to cut out when I hit bumps, I cleaned the electrical contacts on the plug under the hood, and now that's gone away. Still jumpy.

I took the speedo head out to fix the odometer, which was successful, and now the speedo is jumpy at lower speeds, this had zero effect on the cruise control, so I probably broke the damper in the speedo and not the cable. Just noted because the VR sensor for the cruise appears to be in the speedo head.

The service records for the car indicate that the cruise amp has been replaced at least 3 times since 1989.... Maybe something is causing it grief?

Then the last thing that may be the root cause, there's play in the ball/cup throttle linkages between the cruise motor and the Rube Goldberg machine on the valve cover. I did adjust the throttle linkages as per the FSM because I was only getting about 80% throttle, that made zero change in the cruise control.

Maybe it's some combo of the above, I just don't know what the preferred method is of fixing those issues. I searched but couldn't find a lot in the way of tightening up the ball/cup seats.