1994 Pontiac Sunbird 2.0 engine When the car is warming up it has heat but after you drive it for mile you lose heat and it blow cold air have replaced the water pump and thermostat?

You will need to change your thermostat, as it is been stuck open. The thermostat is the piece that control the quantity of cooling fluid that goes to the radiator, if its stuck open all your fluid is going to be cooled by ambiant air on the radiator, and the engine will get colder then its operating temperature. If it close properly it will force the coolant to be cooled by the heater radiator, which transfer the heat to the air in your heater system. The piece itself is like 10$ or less, but the installation can be tricky if your not mechanicly inclined.