Keith - overheating
Have you pushed your engine beyond 2800 for an extended period of time. Being on Lake Ontario I'm assuming you motor a short distance out of your marina and hoist sails, which doesn't give the engine any extended motoring. Typically these will overheat between 3200- 3400 ( Yanmar cruising speed) after a period of time.Mine started last spring, bewteen 60 - 70 hours on the engine, after I was motoring for an hour between 3000 - 3200.