In addition to the above things, it could be a cooling system problem (thermostat too low, stuck open or no thermostat, bad radiator cap), a vacuum leak, or a fuel problem (too much or too little fuel).
We need to know more about the vehicle - make, model, and year would be a good place to start. Also list as much as possible about the truck, like what engine, what transmission, recent work or repairs done to it, etc. How does it run? Good? Crappy? Usually good but suddenly crappy?