Well, the old 2000 F150 has been perking along just fine until the dog days hit around here this past month. The 4.6L V8 has 222k miles on it now, but the overheating problem started a couple of summers ago when I would tow my trailer in 90+ degree weather, never any trouble in any conditions prior to that. Now, it overheats any time I go out in 90+ degrees IF I run the A/C, no problem if I leave the A/C off.
Back when this started a couple of years ago I replaced the following in this order...coolant flush & fill with a new T-stat...no effect; fan clutch & T-stat(again)...no effect; lower radiator hose...no effect.
I bought a new Gates water pump at the time, but just installed it yesterday. I didn't see any signs of wear on the old Motorcraft pump, it actually looks like a better pump than the Gates, longer impeller vanes.
If the new pump made any difference, I'd say the problem is actually worse. So, I guess the only thing left to do is find a radiator shop or order a new one.