Yes there can be a possibility of a thermostat going intermittent, but must remember his Hot water still works fine when presses the +1 hr button, so would that not rule out this. Unless there was intermittent fault or connection to it, these would be very easy to test as these are not thermistor type sensors but a bimetallic thermostat switch with a temperature setting knob or screw, it could be tested easily with a multimeter as it would trip either On or OFF,
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