I have an 04 galant when i am at a stop light and i have the a/c or heater on my car wants to turn off. What can be the problem?

The a/c clutch puts a load on the engine if the system is charged with freon. The engine should have a way of increasing rpm slightly to compensate.

The heater, on the other hand, simply operates a fan and maybe adjusts the door inside the dash board to make air flow to the floor vents.

I think that if the car is this sensitive to a/c or heat, then it needs a tune up.

Also, are you sure it is 'heat' and not 'defrost' that you switch to when it stalls?

Because the a/c compressor goes on when the 'defrost' selection is made, which would point to the actuator or iacv (idle air control valve) is not bumping up the air and fuel mixture to increase the rpm slightly.