I just solved a nagging problem with my old 78 300D. It would heat up at idle and sometimes on the freeways. I sort of tried to ignore it. Was successful for a couple of years. Then I finally pulled the radiator. Noticed there was build up of solid oil/dirt on the external cooling fins. A little spray on with some cleaner and a quick rinse and is good as new.
My kind of repair. Quick and clean, with little cost.