90 yamaha 2 stroke alarm sounding

[FEA111]

not on initial startup in morning but after running 30 or so mintes to first spot & fish for 20 or 30 minutes infrequently after restarting & engine running at idle for more than 5 minutes alarm will sound and will stay on for like 8 t0 15 minutes then go off. water coming good from pee hole & oil filled on every outing & has never gone below half filled.once goes off runs fine at high speed say for 40 minutes back to dock & hit 15 minute no wake zone & drop passenger off to go retrieve trailer & while circling to lauch will go off again.would like any info on troubleshooting tips & how to do it BECAUSE THIS IS AN INTERMITANT PROBLEM AM AFRAID THIS COULD BE VERY COSTLY IF BROUGHT TO MECHANIC & NOTHING FOUND WITH A GUESSING PART REPLACEMENT TAKING PLACE. BOAT USED APPROXIMATELY 10 TIMES A YEAR ON AVERAGE IN SALTWATER & IS FLUSHED RELIGIOULY ON AFTER EACH USE.THANKS IN ADVANCE FOR ANY IN FO

[Andy'sDelight]

Sounds like an exhaust leak. When you're running full speed the air circulates and draws the leaking exhaust out. Once you are at idle speeds for a while the leaking exhaust gets up in the cowling and starts heating up the engine, triggering the overheat alarm.