hotwater system issues

Hi     I have a Hardie Dux HDE250 electric hotwater system, 250L. installed 1993. I have had no problems with it except the the Pressure reducing valve was leaking water so I replaced it. The hot water system accepted max of 680kpa input pressure, the old valve was 0.53l/s at 700kpa, the new valve is 0.50l/s at 500kpa. A plumber who sold me the pressure reducing valve told me the old one was too high(700kpa), and said 500kpa was better for my hotwater inlet. Fine, I replaced it, no more leaking water, but now I have a few more problems since I did this. First night I took a shower the hot water ran out after 2-5mins and took a long time to recover. I then tried again the next day, then it was fine, no problems for 2-3weeks. Till last night, the Temperature Pressure Relief Valve (TPR valve) that goes from heater to drain was making a constant hissing noise all night and the Discharge Tube was hot. I got up the next day to take a shower and the hot water only lasted for 2-5mins and took a long time to recover.
Is he TPR valve stuffed? Are they expensive? The Pressure reducing valve cost me $65 (AU). The Discharge Tube is "always very hot" even when the heater was working fine , should this be the case? If the TPR valve is stuffed, how do I replace it with out burring my self, do I turn off the water mains and hot water power and drain the water from the heater by opening the TPR valve till the pressure in the heater tank goes down? Could this be due to the change of the Pressure reducing valve? I don't see anything on the heater were I can adjust the temperature of the water.
thanks caven
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