Heating problems in Cavalier

I changed the water pump and thermostat and now the heater in the car does not heat up. It will heat up at idle but once it reaches to 50-55 mph the temp. starts to go down and the heat stops working. I have changed the thermostat 2 or 3 times now and the problem is not fixed. I have also put card board in the car as well to block the wind, this has not helped either. Any suggestions would be very helpful!
Thank you!

Have you bled the air out of the system?