So I'm not an expert, but I'll throw a few things into the mix:
The thermostat's job is to restrict cooling water for a while until the engine warms up, after that it opens and allows maximal cooling. This is to speed warm up and no thermostat is designed to close off at cruising or max rpm it's function should not affect your max operating temp.
If you do want to check the thermostat remove it, put in in a pot of water on the
stove and turn it on. Have a thermometer in the pot and record the temp when it pops open.
But, the thermostat isn't the problem is is it? Are you reaching your rated RPM? If you're lugging your engine (running it under rated rpm) you will run cold and have all the problems already listed. Currently we aren't getting up to rated rpm and this summer we're going to troubleshoot this very problem.
Any of these problems can cause you not to reach RMP:
If your
fuel is old or restricted
If your throttle cable is not at full travel
If your prop is too large or has too much pitch
If your racor filter element is too fine
Poor engine/prop selection is often the cause. Even the Dashews recommend a large prop turning slowly for "fuel economy". I've know people who repowered to get this effect at the expense of their engine.
Medsailor