I have a 70's era 20hp Johnson that will not stay in forward. It doesn't seem to be caused by too much or too little throttle just seems to happen randomly. If the shifter lever is held forward it will usually pop back in or stay in by itself. I noticed that when you move the lever, there is a vertical type of bar that moves in or out depending on the shifter position. It also has a spring. I noticed when the motor slips out of forward the vertical piece and the spring kind begin to vibrate a lot. I know that sounds vague, I will try to get a picture of it.
