Sounds very much to me like your thermostat/s is /are faulty and should be checked out by an electrician – they are the devices that control the flow of electric current to the elements – the elements are just the means of providing the heat once they have been told by the thermostat to heat up or shut down. If you are having to press the overheat switch to get the water heater going again then it is almost certain that your thermostat/s are at fault by allowing the water to be overheated and triggering off the overheat device.
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