Couple of possibilities. Main one is water pump failure. They don't have to leak to be bad when the bearing goes they don't pump thus the coolant doesn't circulate causing an overheat. Another possibility is a blockage in the system usually caused by a plugged heater core. If the heater isn't pumping out heat, then the core is plugged and needs backflushed or replaced. I would look into the water pump first. Lastly, does the cooling fan turn on? A bad fan motor won't cool the radiator and cause overheat. If engine cools somewhat when driving but, overheats quickly while idling the fan may not be activating. Check that the fan is running when engine is hot.