+1 on the overheating issue. Check the face of the radiator for cold spots as it should give you an indication of poor coolant flow. I haven't used soda crystals as recommended by tortron but I have filled radiators and heater cores with citric acid solution to de-scale them. Worked very well - I would also heat the core tubes with a heat gun to excite the solution and flush the stuff through the radiator as best as possible before flushing both directions with a garden hose thoroughly (leave the solution in for 1/2-1 hour. Radiator out, lying face down on a flat surface with the cap on, drain, check and repeat as you feel necessary) Straighten any bent fins, strip the radiator with paint stripper and paint it with satin epoxy black paint. It will be as close to new as you will get. The mechanical fan lets the cooling system down and these trucks are prone to heat soak, so any improvement you can make to heat dissipation will help preserve the head.