88' Jeta, Aoutomatic, 4 speed, heater not working.

Hello all, any help on this would be greatly appreciated. I've searched the forums and I saw a lot of people having somewhat the same issues.

Symptoms: Heater does not put out hot air, only cold air. At one time it did however put out luke warm air after driving awhile and coming to a stop, soon as you moved again air would be cold, almost like its constantly getting outside air? No idea. I've replaced t-stat, checked coolant in reserved, car doesn't seem to over heat, and I've attempted to flush out the heater core, it didn't seem very dirty. Is there anything else I should check or look into before taking into a mechanic?

Thanks for any help guys :D