Ensure that you do not have a leak in your pressuer relief valve which is not uncommon. If you have not replaced the anode (which is part of the hot water discharge assembly) and throughly flushed the debris out of the tank, the relief valve may be leaking intermitently with debris occassionally blocking it. When I contacted Raritan about a similar matter the technician I spoke with instructed me to remove the pressure relief valve and see if there is any evidence of debris. Then, remove the anode and use a shop-vac to suction out the debris (it may take quite a few fills/suctions before you get nothing but clear water). I replaced both the pressure relief vale and anode. (Anodes need be replaced at least every 5 years.) The outer shell can actually be replaced if necessary and dry insulation added. (I made a blanket out of "Reflectix").
With the foregoing our heater was restored for about $100 USD or so (the anode was about $62.)
"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."