My heat alarm starts after possibly 10 seconds when I slow to an idle following any normal prolonged run under a load. It also does it after I start it up cold and let it idle for several minutes. The alarm turns off, in possibly 10 seconds, if I increase the speed slightly. I have replaced the impeller twice. (The old ones showed no signs of wear or damage.) I have also replaced the cork gasket on the raw water strainer and the water hose, thinking that it might be sucking air somewhere. The strainer screen is clean and I was quite impressed by the amount of water that came thru the intake when I removed the hose! I have no way of checking the amount of water exiting as the exhaust is on the bottom side of the boat. I would think that if there was an obstruction somewhere, the problem would be more pronounced under load. I not experienced with any problems of thermostats that causes them to only fail to function at idle. I guess that that will be my next try, as maybe the decrease in water flow is not helping it open when it wants to. If anyone has any thoughts, I would appreciate them. (I have even tried re-uphosltering the seat cushion, to no avail.)