Hi there am just wondering my fiesta is overheating but not on idle only about 3-4 miles into journey bout 40-50 mph things i have done to resolve it and i have no heaters

new thermostat
new waterpump
removed rad to check if its blocked
not loosing any water
also back bleeded it through top hose so no air lock
new htc heater controll valve 12 months back

no signs of headgasket plz help