Sounds like your transmission feathering valve (inching valve) stuck open. You probably need to take cover off, remove control valves and give them a good cleaning and polishing. Be careful, there are strict proceedures for removing control cables to get the cover off. The control valve is under the top cover on all but the latest S.O.S. units. You didn't say what year yours is.
You really need a service manual before you tackle this!!!!!!!