How old is that engine ? Used to have a 96 Evi 15 HP with same sypmtons, removed cylinder head, cleaned all internal crankcase and head water passages, placed a new well torqued head gasket + new spark plugs and bingo!! Still runing up to date.
Head gaskets are not eternal, with time and use they compress and hardens, when engine is hot lets small water particles pass into combustion chambers causing strange symptoms at speeds higher than idle, worsens between 3/4 to wot and even can shut engine down.
Nobody retorques head gasket when engine has passed break in period because a torque wrench is needed, this causes premature gasket wear due to salt water penetration & corrosion between crankcase and head walls losing perfect chamber seal as when engine was brand new.
If all related fuel, electrical, mechanical is working ok this is the inmediate part to check and change. Remenber outboards are not car that uses glycol for engine refrigeration. To get the idea...