Car overheats when below 40 deg F

I had this problem last winter: The car will overheat when the outside temperature gets approx. below 40 deg F; but only while idling. In the extreme cold(sub 0 deg F) it will overheat all the time.

As soon as temperatures rose in the spring the problem went away. Then over the summer I replaced the water pump and antifreeze.

Yesterday temps finally went below 40 again and what do you know it happens again.

My only idea is that it’s a sticky thermostat(Firestone brand, 4 years old). It’s not the water pump or incorrectly mixed antifreeze or air bubbles in the system.

Anyone else had this problem?