if your TP valve is working correctly (and by changing them and stillg etting the same reaction we will assume it is) then it is safe to say you are overheating the water or overpressuring the tank. over pressuring setting is normally around 150 psi. just about impossable on well water system, so lets look at the temp. the TP is likley set at 210 degrees so perhaps your aquastat is not working properly or it is located near a cooler spot in your OWB (too close to return water) and in fact you are heating the water to the 210 mark?
your well system should have a pressure gauge on it, you can see right away if the pressure is too high
i am guessing yoru water is too hot (perhaps even 200 may cause the TP valve to open slightly
as a test of the side arm perhaps you could shut the delivery water from the OWB off and see if the leaking stops when the tank cools to the normal setting of the water heater
i tend to agree with CRJR, your water is too hot. for a test why not turn the aquastat down to ON at 170 and OFF at 180 go back to the home and run the hot water untill the valve stops leaking and then wait and see if it does it again