Why is my 2000 ford mustang heat blowning cold but running hot at times?

Why is my 2000 ford mustang heat blowning cold but running hot at times?

Could be a bad heater control valve, clogged heater core, worn out water pump.

First thing to check is the thermostat. If it doesn't open coolant won't circulate through the heater core. Clogged heater core wouldn't cause your engine to overheat. The heater control could be broken too.