DF40 overheating.

[mac105]

Engine runs fine all day up to 4000 revs. 5000 plus after 30 to 45 mins it slows to 4000 revs and sounds as if it was misfiring . There are no lights or alarm. Impellor has been changed also thermostat.Dealer has no idea, can anybody help? I am assuming it is overheating.

[boats.net]

Check compression, have alrm buzzer tested to see if working, test to see if all cylinders are firing when it looses rpm's.