Yamaha f40 runs fine at idle. From the time I put the boat in the water, and idled to the end of the no wake zone was 15 mins. I take off, got up and running @4500 rpms for a mile or so, overheat alarm goes off. The tell tale was flowing fine, water coming out wasn’t hot at all. Impeller was changed last year around May. Before I tear into this, I’m thinking thermostat? Any other thoughts?