OVERHEATING

Many things cause overheating, when you're driving 75 mph you have enough air flow into the radiator the fan is not needed unless driving up a hill. Is the radiator sealed to the core support to force air into the rad and as inertia stated you need to check for any cool spots in the rad with a tool or light touches from fingertips. When you were driving did you hear the fan clutch roaring? I would use a 160 t-stat.