had the same problem last year on my M-18...
cleaned the heat exchanger, replaced the impeller, replaced the intake hose and heat exchanger hoses, new thermostat, new belt...all with no change. Then the raw water pump seals started to drip a little and I bought a new raw water pump for about $125.00, installed it in about 15 minutes and the problem was solved. I think the original 14 year old pump was just worn out of tolerance after 1,000 hours, because now the temp won't move above 165 degrees in warm Florida waters no matter how hard I run it or how hot the weather is. I rebuilt the old pump and keep it as a spare. Hope this helps.