Sorry Carwheel I was responding to the OP's question.
I have a accumulator tank and have never had a problem with the safety valve on the water heater. A accumulator tank has other benefits as well, like the pump doesn't run as often and is quieter. It only takes a few minutes to install. They aren't expensive.
Have you checked the temperature of your water? Not the heater setting but the actual temperature of the water. Does the valve leak when heating with propane or electric or both? If it only leaks when heating with one or the other it may be that thermostat. The propane and electric heating elements have separate thermostats.
Are you using a water pressure regular? If not it may be a city water pressure spike lifting the valve. Remember there is a check valve at the city water inlet, so a pressure spike will be held in your system at the highest pressure reached. A series of spikes could really jack the pressure up.
If you have replaced the safety twice, it is most likely only a simptom of another problem. Keep looking until you find it. You don't want a busted pipe.
http://www.adventurerv.net/shurflo-a...source=Froogle