It might also be a sticking throttle butterfly. Your description shows striking similarities with issue I had here despite being on an entirely different engine. On occasions it would hold for several seconds 30 mph in 4th or 20 mph in 3rd with foot off throttle, but always reverted to idle when l dipped the clutch.


Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone