Do you have a pressure regulator valve? It sounds to me like you have one but do not have a thermal expansion tank. Briefly what happens is when water heats it expands. In a water system without a pressure regulator, the expansion is absorbed by the city water main, but a pressure regulator has a check valve that prevents this from happening. The expansion has to go somewhere, so if there is no thermal expansion tank, the TP does its job when the pressure gets to 150 psi and release some water and relieves the excess pressure. If you have a thermal expansion tank, it may be shot. Some newer water meters have a check valve in them also, so even without a PRV you could have the same problem.