If you've replaced the valve, there could be a couple of problems as LJ and KP said. Pressure is a problem, in addition check valves are a problem. Some areas of the country have check valves on the water systems. When this occurs, the water in the system can not expand and contract. So when the water heats up, it builds up pressure fast in the system. Without a check valve, the water expands back into the city system. No one really knows it is occuring, because the city system has no check valves, and people are constantly using water. It's possible you have a expansion tank somewhere around the water heater that is used to absorb the pressure expansion. This tank could be full of water (instead of air) and need to be drained or the diaphram that separates the air and water could be broken. Since air is compressive and water is not, when pressure builds up it normally compresses the air in the expansion tank...but when the air is gone, the pressure has to come out the Pressure and Temperature relief valve. Just a thought. Check it out.
Raymond Vin Zant