Your car does not overheat while going 70 mph but when you go 75 it overheats why?

I have a 1991 camaro rs mine is doing the same thing after it overheated once. You might want to check your intake manifold that's what i hae to replace

Answer

bad water pump would be my guess or your engine isn't getting enough air to keep it cool or the fan isn't coming on to help the air flow around the block to cool the radiator or the radiator is clogged and isn't putting out enough coolant to the engine or your thermostat is stuck and is definitly not allowing coolant through could be one of 40 different reasons but those are the only ones i can think of at this point.