Hello, It sounds like it’s time to have a look at both elements in the tank. If you’re handy with electricity and have a volt-ohm meter, go here: http://www.waterheaterrescue.com/Troubleshooting/Tanklets/electric-water-heater-issues.html and have a look at all the introductory info and the last question at the bottom. It will tell you how to check elements to see if there is a problem. To me, that overheating might be caused by a leak in the element sheath, creating an element that never really shuts off. Testing with an ohm meter will tell you.
Yours, Larry