My truck temperature tends to rise when go about 70 MPH. I flushed the system, replaced the thermostat and the clutch fan. I also pressure washed the radiator's fins.
Coolant on 1990 Chevrolet 3500 Pickup
Take a flashlight and stare down into your radiator ( with the coolant out ) and see if the core of your radiator is getting plugged.
She may be leaning out at 70 mph do you have a exchaust temp gauge or a turbo presser gauge?