My 318 valiant does this.
I had a brand new 3 core high volume after market radiator, fresh engine, water pump, thermostat etc. everything is new.
It's nowhere near as bad since I changed to a 5 blade fan and put in a fan spacer to get the fan as close to the radiator as is possible.
(this bothers me as the 5 blade fan is HEAVYYY and will undoubtedly be robbing horsepower at high RPM)
My theory is simply that a modified motor will create more heat than a factory standard motor at idle and requires better-than-factory solutions.
One day I will build a fan shroud, swap to a flex fan and add additional electric thermo fans to the frontside of the radiator. In the mean time I just have to remember to not let it sit at idle for extended periods.