lots of things go on when an engine gets hot......including fuel expanding.....i wonder if you have a running lean situation on hand.....so at real hot temps when the engine gets really hot the fuel is in essence thinning out and creating a lean mixture situation. try and see if the same thing happens with the AC on when it is cooler outside or before the engine gets really hot....by the way do you have that stock lower air damn installed underneath the radiator? you would be surprised what a difference that makes in cooling down the engine bay.