2000 Dodge Truck AC blows cold, until stopping

I have a 2000 Ram 1500. 112K miles. AC blew cold air fine all summer. Live in Arizona. In the late December early January, when overnight temps got low in the mid 20"s we started using the heater. All was fine. Now that it is warming up some, and we are now using the AC again, the air blow cold but now when we make a stop at a stop sign or light, the AC blows hot air. We have to turn the AC knob to off and then back to either Max AC or AC to get it to blow cold again. Then it does it the next time we stop. This is getting ridiculous. Any ideas as to why. It does it also if we are using the vent, and the temp control is on cold (blue area of temp control). So it doesn't matter if we use the vent or AC, it is ok until we stop. Then we have to basically reset it. It blow out of the dash vents like it should and not the defrost or foot level so I know the selection of vent, defrost, vent / floor work okay, it just goes from cold air to hot air until we reset it and it is coming out of the correct selected vent we want. Thanks for any help we can get.