Yamaha 40 CV two stroke alarming

[The Dealer]

Having trouble with my Yamaha 40 CV 2 stroke outboard motor
No problems 20kph but anything over that the motor alarms and derates
Took boat out this morning and after one minute problem happened
Water from the telltale was cool.
Turn key off for 2 minutes alarm stops
Drive around for half an hour under 20kph no problems
Take it up above 20ks and it alarms and derates
Impeller changed out 18 months ago and has done less than 12 hours
Overheating / Sensor ???

[99yam40]

water pump not pumping enough water or scale build up in passages
my thoughts are scale

but never hurts to make sure switch and stat function is in spec

[The Dealer]

Thanks for the reply
Will check waterpump and for scale