1992 Honda Civic ~Heater is not always working~

Heater problem
1992 Honda Civic 4 cyl Two Wheel Drive Manual

Hi there The car has just been flushed out like the rad, and heater core, also replaced the theromastat. But once the car reaches a speed of about 80km or so the heat works but when you are driving at a speed of 50km or so the heat wont work. The fan is running but there is no heat, and the tempture gage goes way down to cold. Please help Thank you