WIERD - Forward(tiller) shift problem 6hp

[Kerry Prance]

I have a 05 ( I think) 4 stroke ( 2cyl) 6 hp Yamaha long shaft outboard. Although this engine is 9 years old it has seen very little use and has been properly stored. The motor has a total of less than 10 hours use. I recently swapped this motor to a 14.5 jon boat ( with raised jack plate to accommodate the long shaft). As the motor came with a 3.5 pitch "sail" prop, & was way to slow on that light boat I switched it to a new 8.5 pitch prop. the switch was easy and uneventful. and the new prop got the boat working nicely. Then the problem - on the first time out after the prop change. WHen I put the boat into forward from neutral ( at lowest possible idle speed) It takes a few seconds for the gear to engage. it makes a "racheting" sound and then drops into gear with an audible grind and clunk. Then immediately after power is applied it does something I can not explain. The motor makes a "BUMP" sound/sensation as if the skeg has hit a log or something under water ( first few times I thought that's what happened). The motor then runs fine until it is returned to neutral and then it will do this again. The problem is getting worse, as the rachet -sounding time before engagement is getting longer ( 1-2 seconds to maybe 3 now) and it now does the "Bump" sensation - more than once. I am not stranger to small outboards - having owned many & done most of my own minor ( carbs, impellers, thermostats) repairs. This one has me stumped. It has only done this a total of about 6 times before I realized that I had a problem and trailered it. I did NOT notice the problem last year before I changed the prop ( this spring) but I cannot see where that would cause this???? appreciate any help / input thanks