Overheat!!! 1999 sunfire

heres the whole story. I have a 1999 Pontiac Sunfire 2.2L. A few days ago the coolant low indicator came on and i had placed GM Prestone Dexcool 50/50 in it, i kinda topped it off too much and it overflowed out when i turned the engine on and alot of it leaked out. there was a weird sound that came on but i didnt pay attention. a day later the car started to overheat. and the indicator came back. the coolant has became empty. i placed more in and then 15-20 min later, it overheat again. and the coolant disappeared again. i checked the water pump, it seemed fine, i changed the thermostat which was faulty. yet every time i fill it up it disappears after 15-20 min. there are no visible leaks and i had checked the oil and trans and the exhaust and havent found white smoke. the radiator almost stays cool even at overheating temperature. any ideas?!?!? i am gonna check the spark plug today but if thats not it, i dont kno what else i can do. there is no power loss either which i read elsewhere that can result if it was a head gasket leak.

well, check the thermostat

Sounds like your cooling system is air locked. Check your service manual, you need to bleed the air from your system.

I had a Grand Am that would air lock it you didn't fill it correctly.

Do a search on bleeding air from radiator.

U may possibly have a shot head gasket... my dads grand am did that, i would fill it then it would dissappear and then it overheats. is there a puddle under ur car? also check the plugs, like the threads, if there is white crap on em that may also be an indacation of a blown head gasket. quite a pricy job, cost my leik 2 grand to get it fixed