Although it's a new radiator, has this thing been in the Alabama mud since replacing the radiator? Could the core have a buncha mud stuck in it? How about real dusty trails? Heavy dirt build up on the core can cause it to run hot too. How's the timing on this? Retarded, advanced? Do you get full advance at high speed? All things to check if any of these apply. 
Just a side note....mine ran like this with a blown headgasket with no other signs of trouble for about 3 weeks until it finally got bad enough to tell. 
I was thinking timing and the headgasket was in the back of my mind but I didn't want to just throw that one out there yet until he tried some other things. Anyways for your minds sake now get a leakdown test to make sure the headgasket is ok.
