L4-2.0L,My 2003 Hyundai air conditioning works fine but often starts blowing hot air, especially when idling.

Air conditioning has started to work sometimes and sometimes blows hot air. If the air conditioning is working fine and I have to idle at a stop light, it will almost always start blowing hot air and, usually, won't go back to blowing cold air for the rest of the trip.

My car has 115000 miles.
My car has an automatic transmission.