Probably accessories.Fan speed should not make a difference at 80mph. You already have 80mph of wind blowing on the radiator, so the fan won't be able to pull anymore air at those speeds. It could be a Coolant flow issue such as a clog or restriction or the radiator isn't big enough. If you have a bunch of accessories blocking the grill that might play a part.
Cooling system has been completely redone short of changing hard lines and the heater core.