Are symptoms of a broken heater core constant or do they occur on and off?

You are using the wrong antifreeze!

A "broken" heater core leaks. As the engine gets hot and coolant pressure rises, it will leak worse and worse, which may be when you notice it. Radiator stop leak may work for a while then the heater radiator ("core") may start leaking again. Or the stop leak may fix it. You may smell antifreeze odor inside the car. Don't breathe this stuff - it is deadly poisonous (destroys the liver)ESPECIALLY if vapors are inhaled. If you just HAVE to drive the vehicle to the repair shop, roll all the Windows down and leave the heater/AC system "OFF" and the temperature control set to "COLD."