1979 ford f150 sludge/sediment in cooling system?

My truck runs hot at speeds above 60. Cooling system has brown/red sludge all in it. I have flushed radiator 3-4 times. I have ran radiator cleaner through it. I hve taken the radiator out and flushed it from every opening. I have replaced thermostat. Truck still overheats and has sludge in system.

Believe it or not: Dawn dish-washing detergent will break this sludge down and allow you to get it clean. Run the Dawn for about 100 miles.