The car overheats

Ive replaced the water-pump, taken all hoses off and checked for blockages, checked radiator for blockages, checked everywhere for leaks, checked thermostat, checked radiator fan. Everything works.
After about 30kms of motorway driving, the temp gauge will go up. If I drive with the heater on the temp stays good/in the middle.
I have also done an oil change and have not noticed any water in the oil.
Help please
