Are you displacing any coolant from the radiator(pushing coolant out of the radiator and overflowing the reservoir)? Are you spewing white smoke out your exhaust? If not, I would wager thermostat before I would consider the hgeadgasket. Thermostat is an easier fix too, just pull it out, and suspend it in a container of water with a thermometer. Heat up the container while watching the thermometer and see when the thermostat starts to open, if at all.