Many of the radiators I determined to have caused overheating had accumulated huge amounts of debris and field dust turned to dried mud due to areas of damaged cooling fins on the fan side of the radiator.
The cooling fins soldered to the tubes dissipate heat just like the cooling fins on a small engine requiring them to be clean and have airflow along them to be efficient.
When these fan-side fins are bent over, not only is airflow reduced but they also trap debris and dust which fill the core from the inside out, further impeding airflow through the core.
Remember the dirt that collects in the air cleaner cup was also drawn from the same dust cloud inhaled by the fan, whether it passed through or became lodged depends on cooling fin condition.
Damage to the fan side of the radiator is most often caused while changing belts. Before performing mechanical work near the radiator, cut a piece of cardboard to cover the core to ensure knuckles, belts or wrenches don't flatten the fragile fins.
If fins are found to be damaged, obtain a fin straightening tool or other suitable device to correct the misaligned fins. With time and patience, you can transform flattened fins to near original alignment and spacing. The straightening tool does a nice job but you must start the straightening process one fin at a time before the tool will dig in.
If you use compressed air, it is better directed from the fan side pushing the crud out the way it went in. Unless you are positive the cooling fins are straight on the inside, avoid air or water from the front as the debris has no where to go and will only pack tighter in the damaged areas further reducing radiator efficiency.
Sometimes the simplest causes of overheating are overlooked and not addressed because they are not in plain sight.