I have 2002 f 350 diesel overheats whan I driving over 100km once I slow down to80km it runs fineanybody have clue

I suspect phosphate (mineral) scale has built-up in the radiator, which is blocking many passages. You will likely have to replace the radiator. Completely drain the system and refill the new radiator with a 50/50 mixture of the recommended antifreeze (likely G-05) and distilled water. Be sure to remove the hex-head drain plugs, located on each side of the engine block, after disconnecting the negative battery cable. This is a must on diesel engines. The distilled water will prevent phosphate scale from forming in the future. Add 8 to 10 ounces of Supplemental Cooling Additive (SCA) every 15,000 miles, testing it with Fleetguard strips every other month. Completely drain and refill the cooling system with a 50/50 mixture of approved antifreeze and distilled water every 30,000 miles. The SCA will help prevent cavitation, which is a big deal on diesel engines.