Make sure that your cooling system is coming up to operating pressure. When cold, squeeze radiator hose to see how it feels with no pressure. Start it up and let it run 15-20 minutes, till coolant temp is ~195deg. (stock thermostat temp). Squeeze the radiator hose, should be under pressure. No pressure = leak somewhere, and your coolant will boil out (coolant boils at lower temp when not under pressure). Usually it's the coolant reservior cap that leaks, its supposed to hold up to 15psi. You can get a replacement ~$5 at parts store. Sometimes it's the reservoir itself that won't hold the pressure.
Hope this helps.