I read your thread a little too late to have you check the radiator temp in all of the location to see if there were any colder or lower temperatures in any location. I had an overheat issue with my Road Runner and went through pretty much the same checks as you, even flushing the radiator. I'm at my friends place and just pulled into his driveway and had told him about the overheating issues and what I had tried. He walks back into his garage and comes out with his infrared heat gunn, opens my hood and within a matter of seconds shows me how 75% of my radiator is almost ambient temperature and the other 25% is really hot. "plugged" radiator down inside the veins where you really couldn't see even with a scope. With the radiator out I don't know if you can fill it with hot water and find where it doesn't raise the radiator temp or not. It was real obvious with engine running to see the difference in radiator (coolant) temperature. Had me fooled as the radiator was restored on the outside and looking new but was badly plugged and flow restricted on the inside.....new radiator core fixed it as it was so badly plugged. Oh and there is a method to check the coolant to see if there is a compression leak getting into the coolant.......works "most" of the time.