I had that problem once.... i cracked my radiator and didnt know it so about a week or so i kept having to add water. so then the crack got worse and i noticed it. so i changed the radiator, samco hoses, thermostat housing, and thermostat( it was sticking and thats how my radiator cracked)... so when i put the new thermostat in the gasket didnt sit right and for weeks i kept losing water so i had to take it back off which is the biggest pain in the ass, and i put high temp sealent and put it on right.. and havent had a problem since... you might want to check all your sensors, radiator fan switch, anything that could be causing it to over heat