I installed a new radiator but when I climb a hill or mountain in 90 degree + heat the cooling system heats up. If I turn off the a/c and turn on the heater full it usually cools back to normal. on any other terrain other than a steep hill, it stays in the normal range with the a/c on. It doesn’t leak or use any water (no leaks). Only in hot weather and on an incline.
No more hills for you, Just kidding. Sounds like the thermostat may be sticking open a bit. When this happens the coolant doesn’t stay in the radiator long enough to cool under stressful conditions such as heavy acceleration or climbing hills.