It sounds like the thermostat is not working and it is overheating the water. This could be why the relief valve is "leaking", and the unit is shutting down, until cool. Does the hot water seem REALLY hot? Does the unit have electronic controls, or is it a standard pilot ignition type water heater?
2003 GMC 2500HD 4X4 D/A Ext. Cab
Propane Powered Honda EU2000i
Lots of Hot Sauce!
Air # 283