radiator ?

I drove the car several miles noticed steam coming from hood. There was steam coming out of radiator. I stop and filled it with water and drove twenty or so miles no steam. I ran the defrost so the fan would stay on while driving. I stop half way home at a radiator shop they did not look at the leak but said to buy a new. I checked before leaving the place and radiator was not leaking. About two miles from my place I turned the heater on and when I got home the radiator was leaking again not as bad as before but still leaking. There does not seem to be a radiator on the market now. Can someone tell why it quit leaking with the fan running and start again with the heater on. thanks