40TLRC continues to derate

2004 Yamaha 2 stroke 40TLRC. Engine derates to idle. Completely replaced and/or rebuilt fuel system from fuel tank to reed valves. Problem only got worse; engine would run for five minutes and then derate to idle and then die. Replaced plugs, ignition coils, no help. Replaced CDI and problem went away for about ten hours of use. Now when running at full throttle engine runs great for fifteen minutes and then derates to an idle. Replaced impeller and thermostat and ensures water passages were clear. Engine is cooling fine but continues to derate after fifteen minutes at full throttle. Starting to suspect bad engine overheat sensors or something wrong with the stator or I was sold a bad CDI. Thoughts?