273 overheating after new timing chain set and waterpump

My 69 Dart has the original 273 radiator in it with a 360 in it now with factory A/C. My car would do the same thing, cool great as long as I was moving, but slowly climb in temperature when I was at a red light. The radiator looked very clean but I pulled it out and took it to a old school radiator shop that removed the tanks and Rodded it out. Man what a difference, it cools in traffic even with the A/C on. The guy at the radiator shop did tell me it was pretty clogged up.