Absolutely 100% correct. years ago, 1989 to be exact, my in-laws asked my wife to drive one of their cars from N. Virginia to S. Carolina. My father-in-law, who was a total jerk about car maintenance, took the car to his shop to have it checked out. The shop said it was 100% ready for the trip. Now this was a 10 year old car my mother in law drove 2 miles every day back and forth to work. It had less than 20K miles on it. My wife hadn't gone 40 miles before she was calling me to tell me the car was overheating. I had to go rescue her, and when I got there I could not believe the condition of the radiator-solid rust. It was fine for a daily drive of 1 mile each way @ 25 mph. But on the highway @ 55 the cooling system was just too clogged with rust. Had to have it towed to a Ford dealer where the radiator was replaced as well as all hoses and a flush of the cooling system, to great expense of my idiot father-in-law. I also took great pleasure in telling him his "trusted shop" he went to for years were crooks, as there was no way anyone had looked at that radiator. I wanted to push that damn car off a cliff for all the aggravation it caused me! 

Jeff Stewart 

----- Original Message -----

A clogged radiator, short of total blockage, may provide just enough 
cooling under light to moderate loads and vehicle moving and pushing air 
through the radiator. But under less than ideal conditions, like stopped 
in traffic or climbing mountain passes etc. heat builds to the point of 
overheating. Or when engine is shut off and coolant not being circulated 
the already to hot engine will boil the coolant, thus the bubbles and 
overflow.