1999 Hyundai Elantra transmission issues

I have a car with transmission issues. The original transmission totally blew 3 years ago and it was rebuilt. It always tended to stall upon stopping at least 3 or 4 times during any given trip. Problem has gotten worse. Upon stopping the car at a light or for any reason it stalls constantly until it warms up for 15 or 20 minutes, then problem alleviates. I don't want to take it in to the shop and have mechanic overcharge me for not knowing what I am talking about. If I just ran engine for 15 minutes to warm things up all the time would that solve problem? What is going on?