My 1994 ford explorer (166,000mi) runs great during colder months, whether city or hwy and upgrades or downgrades. During warmer months, since last year and now this year, it loses power (from 65-45,in seconds) while going on slight upgrades at speeds above 45mph, with the gas pedal floored. I can not increase speed, unless downshift into lower gear (auto tran), or until road levels out again. Not sure if tranny, oxygen sensor, or vacuum problem, since only occurs during warmer outside temperatures.

I think there is a fuel problem , check fuel flow and pressure, if the flow is low replace the fuel filter, if pressure is low check fuel pressure regulator if ok replace fuel pump.

Thanks for giving a good deal of info to go on. When i think of what temperature will have an effect on I think of oil. Oil would work better in winter because it is thicker and lubricates better. In summer it will thin out and maybe with the higher mileage it has an effect on the motor power. What oil was used on the last oil change. Most popular is a 10w-30w. Maybe summer would be better to use straight 30w. If this helps please reply and good luck