Just out of curiosity,
Do you have a seal on the bottom of the hood that seals against the radiator support? I have found that when idling with the hood open there's no issue and the engine will run at a reasonable 180-190° BUT if that seal isn't present what happens is heat from under the hood escapes from under the hood lip and it gets sucked back through the radiator and gets heated up and then escapes from under the hood lip again and.....well you get the picture. I have a fiberglass hood so I didn't have the seal. I wound up buying foam rubber from Home Depot and I glued it to the bottom of the hood so it sealed to the rad support. Huge difference. You can try a zero dollar test and put a couple of bath towels in-between the hood and the rad support. Just a suggestion, good luck!
