did you put any radiator stop leak in the radiator after it started leaking the first time? where exactly is the coolant leaking from? is it just boiling out of the bottle or is it coming from elseware on the motor. you could have a clogged heater core or a bad radiator. if you dont change your coolant this will happen. I belive you have Dexcool antifreeze/coolant in your cooling system, its the orange stuff. that should be changed every 5 years or 150,000 miles according to the manufacture. but in reality the stuff is complete garbage, basically turns to mud in your cooling system. GM is now changing out Dexcool with standard ethlyline glycol, which is the green stuff because of all the problems. it is possible this mud buildup is clogging one of your cooling tubes, heater core, or radiator.