Like Techinspector, I live in AZ (fortunately a little cooler part, it only gets to about 110 here in the summer).

Under the conditions you describe, I tend to agree that the problem is lack of airflow at low speed. My cure in the past has been to use a good 7-8 blade fixed fan with a fair amount of pitch and the heavy duty (about 2 inch thick and twice the price of the standard duty) 454 Truck clutch. They roar a little at speed when they lock up (when the engine starts to heat), but it usually cures the problem.