one other idea. Replace the water pump. Or at least remove it and inspect the impeller. I've seen these things where the impeller has eroded (thru electrolysis caused by letting the antifreeze turn into an acidic mess) enough that it barely moves any water. Just replaced one for a friend on a 4.6 truck. Ran normal temps until the recent heat wave, then it would overheat. Removed the water pump and the impeller was almost completely gone. Ifyou have seen one, each blade has a 90 degree bend in it to sling the water. Those blades were gone at the bend point, and what was left of the flat part was incredibly thin. The 4.6 water pump is a snap to replace, and is not very expensive.
