Chevrolet: Heater problems 1997 Chevy Cavalier, 1997 chevy cavalier, high gears

Question
Two months ago had head gasket replaced, radiator flushed and filled and new thermo put in. When car is started in the AM, car heat works, when you put the car in gear and drive, temp goes down and heater blows out cold air. When you park the car and turn heater off, leaving car run, heat goes back up. Also car started missing low and high gears. Does heater sound like air bubble is in the system? Any info is greatly appreciated.  

Answer
Hi Craig;
       Have you checked the radiator when warm to see if it is full /also how is the rad. cap ? and is the rad. holding the p.s.i. re-check the coolent and fill if needed also check to see if it is holding the pressure if not ,there is the trouble.