Check pressure regulator in front of house to make sure it is not gone bad and is allowing too much pressure into house. The relief valve at the water heater is also for temperature. The thermostat at water heater could be bad. Look at the water heater for manufacture date. If it is older then ten years it may be time for new heater. Sometimes you can change the temperature and pressure relief valve if it has gone bad, but it is possible that the brand new one will also leak if that was not the problem. Also if your water heater is in the house you should run a pipe from the pressure relief to the outside of the house were you would see it if it leaked in the future. Good Luck
.
