105 series cruise control

Check your brake lights and park lights. If a globe is faulty or broken filament touching the park filament or similar, it sends a voltage to the brake light switch and appears to the cruise system as though you are braking. Check globes and the base connection in the holder plus a lok for everything being proper in the trailer plug and base.

Martian

In addition to checking that all the lights are FULLY functional (check brake lights with tail lights already lighted and look for the change in intensity) I would also be looking at the Bowden cable from the CC motor to the engine throttle plate.

I have been able to have these cables made up by visiting the local boating supplier, as they use similar cables for some of their outboard motor controls.

Regards
Athol