Sounds like either the internal water flow bypass is stuck, diverting too much of the water flow away from the heat exchanger or the heat exchanger is choked with scale. Both case prevent sufficient water to pass through the heat exchanger. Water starts to boil.

Another potential is the pump is sucking too much air instead of water. Is there much air in the pump when it's running?

Scott

Scott,
Once again thanks for your quick response. Husband packed the valves which eliminated the air in the pump. So far so good.