Mustangs always seem to run hot, like really hot. My Mach will show the temp gauge needle straight up all the time, winter or summer. But it definitely seems to be down on power when it's 80 and real humid or over 85 outside. It never over heats, but you can tell there's some heat soak going on. Runs like a dog with the AC on too. Best thing you can do is make sure your radiator is clear on the outside of debris and the fans work. Make sure the radiator fluid is fresh every couple years and no pressure leaks. Higher heat outside can also make pre-ignition/knocking/pinging etc more prevalent if your tune is too aggressive. That's the problems I had with AM.