If you have an expansion tank (you should) it is shot, replace it. If you don't have one, put one in. The PRV makes the house a closed system - water comes in , but can't get back out. WHen you use water from the hot side, it is replaced with denser cold. Then it gets heated and expands - it has to go somewhere - basically, it increases the pressure until something gives. An expansion tank gives it someplace to go while keeping the pressure in line.