Agreed.
I had a similar problem with mine. Even with the aftermarket radiator, my 360 would slowly overheat in heavy traffic or standing still. It was only going at 30 mph+ that it would start cooling down.
My 360 has 10.2 compression with a 750 Street Demon carb (with the phenolic fuel bowl) on top of an Edel Air-gap intake. So I had goodies to help cool the fuel down.
The low speed/idle slow overheating issue was resolved once I put a fan shroud on.