2002 Mercury 50HP 2 Cycle

OK. long story. A year ago last fall I bought a 24ft Fisher pontoon boat with a Mercury 50HP engine. I got a deal because the voltage regulator had shorted out and burned up all the wiring around it. I pulled the motor and repaired it over last winter. I replaced the wiring, the stator, igniter, water pump impeller, rebuilt the fuel pump, replaced the fuel line and pump ball from the gas tank and cleaned the carbs. It ran great for the first few times we were out, but one afternoon we were running along at full speed and suddenly it dropped to about 2/3 power and I have not been able to figure out what is wrong. Compression is 110 cross all 3 cylinders. I replaced the plug wires on each CDM and even bought another CDM to swap the other three out one at a time to see if one was bad. I have also replaced the fuel filter, plugs and checked the throttle linkage. I have also checked for spark on each cylinder with my timing light and all 3 had spark. At the end of the season last summer, I disconnected the black/yellow wire from the rev limiter in case it was bad. No change. Yesterday I was driving as fast as it would go, I thought that if it was lack of fuel I could hit the starting enrichment button and see if it would help. Nope, it made it bog down until the fuel was cleared. I am at a loss as to what it could be. Anybody here have any ideas as to what it could be? Thanks in advance.