ford iveco 17 ton keeps over heating ?

when i am driving fast or slow my ford iveco 17 tonner over heats i have flushed through the radiator and put in water coolant i have also checked the thermostate and pressure and it is all good. It over heats mainly when i have a full load which is 8 ton, when over heating occures a red light comes on i stop and let it cool down until i can drive again. How would i resolve or fix ths problem.