well, you might notice, its only cools when you are moving??? then starts to overheat at red light and stops?
Firstly, if the truck has been running since you flushed it, then double check your rad fluid in case there was air pockets. If your rad is absolutley full, expansion tank less then half full, then make sure your radiator fan is coming on, otherwise the fan clutch is shot.
My next guess is your water pump. The pump sapposed to move the coolant throughout the engine block, and too the heater core(heat).