I agree airflow is a likely culprit with the circumstance you describe. But not for certain:

If it has an automatic trans, the heat generated from the converter due to the high idle of a cammed up engine and/or a loose converter can cause the coolant temp to rise from the trans cooler in the radiator. I recommend testing by installing a remote trans cooler and disconnecting the one in the radiator. Or you can drive in traffic and keep popping it into neutral when you are on the brakes to reduce heat from the trans and see if that helps.

Also, what is your timing at idle and off idle, at the RPM that the overheating occurs? It should be at least 20, preferably more, and if you have a operating vacuum advance hooked up to manifold vacuum, that helps too.