Mercruiser 228 Rough Running

I have a Mercruiser 228 installed in a 1980 Searay 225 hardtop. I went through the service manual and adjusted everything into specs. Dwell, Points, timing, tach and air/fuel and it idles like a new engine in the driveway. When I put it in the water, it runs like a champ at WOT. When I shut it down for a few minutes, the problems begin. It will start and idle but when I put it in gear, it spits and sputters. Eventually, it will only idle and dies as soon as i drop it in gear. After a few minutes of that, it eventually wont idle at all and then wont start. Now, If I rotate the distributor, the boat will start and run fine again but its obviously not timed correctly. What in the world can this be? I double checked the firing order, spark plugs, wires, cap and rotor. This has been plaguing me for two seasons and I know it has to be something simple. ANY help is greatly appreciated.