daphatch, the thermostat may be faulty and not opening completely. I have experienced sudden, consistent overheating for no apparent reason. Replacing the thermostat fixed the problem. You can remove the thermostat and place it in a pan of boiling water to check that it opens completely.

You are likely aware when troubleshooting it's best to start with the simple, less-expensive, probable causes and go from there.

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Fred Hoelzle