nigs

my fiesta rs turbo keeps overheating mainly on motorway driveing.there is no water leaks its not low on water and i know the electric fan is working.as the temperature rises at a constant motorway speed the fan starts to blow colder air.would it be a blockage?air lock?knackered thermastat?or could it be the water pump?dont think its the head gasket as no oil is mixing with the water it s not as bad when driveing round town just driveing at a constant high speed.anyone got any ideas????