Overheating at Top Revs

[fishingaz]

hi and all thoughts and ideas appreciated.
i have a 85hp nanni in a arvor 20.i have had the heat exchanger and tubes replaced recently .
now here is my problem everything runs fine when at idle and can troll for 4hours on the fishing grounds .but when i take revs up to max 2800 on way home about 15 to 20 mins the high temp alarm comes on .then i back off to idle alarm goes off within 2 mins.
im guessing exhaust elbow ? but can some of you experts throw some of your knowledge my way.
thanks in advance.
gary.