i had the same problem, turned out to be the heater core hoses were shot, all soggy and stretched out. they no longer would hold when the car got warm. if you feel the black pad over the fire wall under where the hoses are, it should be wet, the leak runs down behind the pad on the fire wall and drops out under the car dead center.
if this is your problem, take the car to the parts store, and test fit the hoses, they let me test fit 10 or so hoses, one side is fatter then the other so its a odd hose. i even had to trim the one i bought a bit to fit right.
