Have you checked the temp sender and gauge to be sure they are not lying to you?
Also... make sure you have good coolant flow. The thermostat does not seem to be the issue since you just replaced it and the problem is still there.
My brother had an '85ish Olds Cutlass with a 305 that had a mysterious overheating issue at higher speeds. It turns out that it was the vanes on the water pump... I don't know the specifics of how it happened, but they were basically no good (rusted out or otherwise inoperative) and the car was barely getting any water flow. At idle it was ok, but on hotter days in the desert it would overheat badly, especially climbing hills (and there were several 7% plus grades he had to climb in very high desert temperatures near Palm Springs fairly often).
The old stand-by test for this is to squeeze the upper radiator hose after the car has warmed up (thermostat open) and see how much resistance you get.
If it was the fan clutch, you would expect to see normal temps at speed and high temps at idle and low speeds, like at stoplights and in heavy traffic.