1997 Ford Aerostar Almost overheating on hot days

Dear Sirs, My van consistently gets to the upper end of the temp gauge when driving with the A/C on flat grade when the outside temp gets above 100, without the A/C it stays around the middle but will get to the upper end when I climb hills, it has not gotten to the red yet, but I don't push it either. It cools downs to operating temp in about five minutes at an idle. Normal operating temp is in-between the N & the O. Has new heater core and valve, new radiator, new water pump, hoses and thermostat, flushed the engine with prestone, new transmission filter and fluid, spark plugs. Cked block for gas leakage, new radiator cap, drive belt (serpentine), new clutch fan also. A/C works well unless the engine has a load on it then it seems to just idle until the load is taken away (let off the gas) then it blows nicely. Was 115 yesterday and I couldn't use the A/C because it took the engine temp too close to the red line. This is my work van that I drive a bunch on the highway and in the mountains, my equipment I haul doesn't weigh more than 500#'s. Any assistance would be awesome and greatly appreciated, Sincerely, Greg