When I stop at a stop light the heater blows cold air, when I start up it gets warm again. Any ideas?

 

i would frist try a thermostat- feel top rad hose after car is warmed up if its only luke warm or cold good chance thermostat is sticking- and while just idling its not pushing enough hot coolant -but at higher rev it forces enough trough to get some heat

 

I have the same problem with my 2002 elantra, but it also has the same problem for the A/C in the summer. In both cases everything is fine, until I stop or am coasting or something that brings the engine RPM's down and then I just get outside air - cold in the winter, hot in the summer. If I rev up the motor then things work as expected. My mechanic says the two systems, heat and A/C, are separate and unrelated, but it seems to me that these problems are connected somehow. Any ideas?