Inside of the shifter is a neutral safety switch (or NSS for short). When you turn the key to start, voltage leaves the key switch, runs through the NSS in the shifter, and then down to the engines slave solenoid, ad this is what cranks the engine over. There are 2 things that go bad here. The first is the NSS is a switch, and just like any other switch it will eventually start to burn out. The 2nd is inside of the shifter there is a mechanism that consists of rollers that ride in grooves, bushings, springs and all of these parts do eventually wear and tear out. What also happens is the part of the mechanism that activates the NSS wears out. The switch itself might be fine, but the portion of the mechanism that activates the switch no longer activates it 100% reliably because of the play and slop that develops inside of the shifter over time.
You have 2 ways you can go on this. You can replace the NSS and see what happens, it may or may not fix the problem. Or you can replace the entire shifter, which will come with a new NSS inside of it, and no matter what might be wrong with your shifter this will obviously fix it. In the shop, we would replace the shifter, it's simply more cost effective to do it that way when the customer is paying for both parts and labor. If your labor is free, it will take roughly 1 to 1.5 hours to get that little switch inside of it changed out if you want to roll the dice and try replacing the actual switch instead of the whole shifter. The other thing is the shifter is a wear and tear item, it is not designed to last the life of the boat.
You are a member so you know how we work, if you had any further questions on the issue by all means feel free to ask. If not, kindly select a rating for my help.
Thanks in advance and good luck!