I have a 3.0 3Vze 4Runner and when I am driving at freeway speeds at about 70mph in the desert (95+ degrees outside) it gets hot while driving up hills. especially when you down shift. If I drive slow 40-50mph it doesn't get hot. The transmission is not overheating (no blinking A/T light). No loss of radiator fluid. Recently replaced thermostat and Radiator. Radiator is a Aluma-Plastic from autozone. I don't believe I have a head gasket leak because I have no loss of fluid and no oil in the radiator. I have burped the system again the other day. When I am driving with outside temp of 60-80 degrees its fine. Any Idea of what is wrong? Bad Radiator?