I'm betting actuator.

The amp has an inclination to "let go" of cruise when digital or analog conditions aren't right.

If the feedback potentiometer in the actuator is sketchy (drops signal, skips across dead spots, etc), or the actuator motor drive is not getting the lever to its intended position, the the amp reads these things as "not right" and drops cruise.

Lube up the mechanisms on the actuator first, see if something is binding. That would be the first step, especially if a "bumpy road" is causing the drop.

Of course the amp itself can have a poor solder joint or connection pin. Which over a bump would cause same thing, so it could be amp. Or for that matter, connector at the actuator. Ensure the actuator connector is not loose / frayed / dirty / etc.

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Cheers!
Scott McPhee

1987 300D