Are you talking about a Water heater if so call the MG. and tell them they might have forgotten to insulate it, is it Res. are com. tell you more then.
Just a thought here. Are you sure the thermostat is actually working once the water initially reaches the preset temperature? Perhaps the power supply to the thermostat and or element is wired incorrectly.
Selgas Services Ltd Craftsman Gasfitters, Plumbers, Electrical Service Technicians