A quick tip, if you hadn't already, check the radiator external cooling fins for blockage (behind the a/c condenser unit). Seems that airborne tree fuzz, sand, and other debris get sucked through the forward-most(a/c)fins and stuck in the radiator.
Keep in mind, the centrifigal, belt run, cooling fan clutch is thermostatically engaged via the bi-metallic spring. If there is minimal hot airflow across the radiator, the fan clutch may not fully engage.