oil fired water heater probs?

Hello, my oil fired water heater has recently started leaking through the air pressure relief valve as if the tank is overfilled and draining through that valve. The only way I've found to stop it is to shut off the jet pump from the well, but then of coarse there's no water to the entire house. Do I need a new water heater or does it sound like a fixable thing a plumber can handle? thanx in advance.