Good question and I am afraid I dont have an answer. Did the water heater rep. say anything about why he thoughht his relief valve leakes..and have you advised him that the new one he installed is leaking also...and see what he suggest....????
Did you go to the web address I suggested ..and was it helpful or informative...?
I still thhink it is related to your high source pressure...my guess is that it shhould not be more than 60 psi...but you should check with your city water service...maybe getting the city to install a pressure regulator at the meter might work!!!