I had a Yamaha 50 a few years ago. There was a friction adjustment on
the control unit. It was just a screw on the aft end of the engine
control, behind the control lever. On my new Yamaha, the screw is on
the side of the control, just below the lever. If you find the
adjustment screw on your control and tighen it up a bit, you shouldn't
have any more trouble.