Is the heater core bad, the water pump or head gasket?

I'm losing antifreeze. Traveled 30 miles to work.  25 miles into the ride the thermastat rose nearly to the H.  It was about a minute after I turned on the heat and then traffic came to a stop on PA tpke. I turned off heater. Traffic was stop and go for next 5 miles. Thermastat stayed about 3/4 of the way up and crept a little closer to the h a couple times. After I got to work, Thank God, I noticed antifreeze reserve tank was empty. Waited a couple of hours and then filled with gallon of antifreeze and a gallon of water. On way home the thermastat stayed at 1/2 way, where it normally is. I did not turn on the heater at all.  I did smell some antifreeze though. And when I got home I added another 2 gallons of water. I noticed it leak once while looking at it for over an hour with my cousin, a mechanic.  It only leaked once and it was after I turned on the heater. But I turned off the heater and it stopped leaking.  We didn't see a leak for the next half hour, even after turning the heater on again and driving it around and going up to 60 mph. I drove home from my cousins for another 5 miles and still no leak.  Is it heater core?  Is it head gasket (engine doesn't really misfire but does seem to have a vibration) is it water pump?  Can't tell where it is leaking but it is toward the gack and side where water pump and heater core are and sort of behind the engine. No visible hose leaks.  162K miles 2003 Mercury Sable 6 cylinder. Again heater was not on at all when I traveled home some 30 miles and still lost 2 gallons of fluid.