You can tell by the rings on your head that it was at one time properly seated, then the soot filled in over it.

I would def say cooling malfunction, but I wonder what part age plays in the warpage of these heads - repeated expansion and contraction.

I don't think idling your car cools the engine, driving on the highway at a constant speed, low load provides the most air flow to heat ratio.
Hot coolant remains in the head (or it should with a proper system) after shut down, it isnt like sending cold water through there.

__________________