I know that the Hummers in the Army have their radiators tilted also and it keeps them running at the normal temp so the tilt aint the problem. What I am thinking is when you have the hood one it is not getting enough air thru the radiator to cool it down enough. Have you tried to see if it over heats with the hood off?? You might have to make a shroud or air dam to direct the air up and thru the radiator from the front.