Scott, You should start by removing your thermostat, check to see if opening. If you have a manual, it will tell you what it opens at, and how to test it. Know they aren't real expensive, but we want to know if it is the problem. yes, if you swap the thermostat and it doesn't overheat, that could also tell you it was the problem.

Another way to check is with a digital thermal gauge ($15), you aim it at different areas of the cylinders/ heads, and if all cylinders heat up equally, then change thermostat. repeat the thermal tests of the cylinders and head temps. If water is flowing, and none of your cylinders are getting too hot. Then you're probably fine.if still getting temps to high on any cylinders, you need to run some salt away, or other type flush to try and remove corrosion blockage.

Good luck, let us know what you find.