At highway speeds (+45mph) the engine cooling fan is not needed, the air current should be enough to keep thinks cool. So that makes think that it could be your water pump, the high RPM's might be enough to overcome a slipping/ broken impeller.
Changing the thermostat is very easy and inexpensive, so that might be worth a shot.
When it was overheating was your heater blowing warm air?
Reason I ask is because typically when a head gasket fails the heater will quit working first because the combustion gasses will get trapped in the heater core.
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