4.3 Merc will not start

[Sea Buzzard]

First visit to iboats. Need some help. I have a 2006 Trophy 2052 with a 4.3 Mercruiser. Last year while running about 4400 rpms the motor slowed to 4000 for about 5 seconds, then picked back up. A little while later I would hear the occasional knock. I got home and put it on the hose and it ran fine. I changed the plugs and it would not start. It would attempt to run but would not. I burned up the starter, had it replaced by the dealer, and the boat ran fine for the one day I was able to take it out after that. It sat all winter long and I got it started and ran it up to operating temperature this spring. I shut it off and have not been able to start it again. The fuel was 2 years old with StaBil added. I syphoned all of the fuel and put new in along with Seafoam and it still will not start. It will turn over, but then tries to diesel over until I turn the key all the way off. All connections appear to be good. All wiring is connected. All circuit breakers/fuses are good. Spark is good to the plug, but have not pulled the plugs yet. Running out of simple things to check and am afraid it is about to get expensive.