Recently discovered when trying ro dock that my transmission lever would engage the transmission but when I released it it would pop back into neutral. If I hold the lever in place the transmission seems to work fine. So far this hasn't occurred in reverse. I have a 1977 Hunter 27 with a tiller. The transmission and throttle are mounted on the side of the cockpit with the transmission furthest out and the throttle towards the centerline. With cable connecting them to their respective places on the engine. The engine is an Yanmar SVE. If you can shed any light on what may be the problem and how to fix it much appreciated.