Pontiac Repair: car heater, pontiac grand prix, car heater

Question
Pontiac Grand Prix.  The heater works fine until the temperature gets below 20 degrees.  Then it blows barely blows warm air.  Already replaced the thermostat. Already had it flushed last winter (twice) and it worked a few weeks and then went back to only blowing cool/barely warm air.  Mechanic suggested to replace the heater core but they way would the heater work great now when the temperature is in the 40's?

Answer
Hi Connie:
I would have the heater core back flushed. before I replaced it. does the temp gage act normal meaning the engine comes up to tempature? if the gage act normal I wouldn't go after a thermostat again. pretty much nothing goes wrong with those cars unless the cooling system got really dirty and thenat may have plugged up the heater core and the flow of coolant is restricted due to the dirt stuck in the core.