2000 Chevy Venture heat

Have the cooling system pressure tested by a mechanic you trust. This will eliminate the possibility of a cracked head or internal leak. Most likely it is not the heater core, but perhaps a blend door in the system, or an improper mix of antifreeze/coolant. How do you know it is not the thermostat? The climate control systems on newer vehicles are quite complex and you will likely need to have a mechanic diagnose this problem to be certain.