my girlfriend has a 2000 monte carlo with a supercharged series 2 supposedly from a 2003 Grand prix gtp. anyway she has been having this problem where it runs fine then overheats. What happened was in the winter she put water in and it got really cold and i think broke the radiator, so after putting in stop leak it worked a while until it started leaking again so i replaced the radiator and noticed it leaking out the old top hose, so i cut it down and it fits fine NO MORE LEAKS. probably that was the whole problem, i replaced it anyway. idk ill Check the old radiator for leaks. so nevermind that, read further below..

so after replacing the thermostat and cap and flushing the coolant and burping the air and everything, it over Pressurizes and coolant overflows out the reservoir tank, then it overheats even if it doesnt say low coolant. the hose will be rock hard and out of the air bleed valve comes either steam or air, not sure because its hot when i do it. where could air be coming in if its not headgasket?

i dont notice any white smoke, bubbles in radiator, or anything that resembles if its the headgasket, but it acts like the thermostat stays closed, then opens randomly, sometimes after going past 195, 202f or 230+ .
anyone know what else could it be? could air still be in the system? am i overfilling coolant? do i need to reflush cause i know some old coolant stayed? what makes it go so high a pressure it pushes it out? i even have a higher pressure cap on too.

There is a crack somewhere, due to putting water in the vehicle.......

Naturally, if no external leaks it is internal......

What I would do is, get the engine warm.......then safely remove the radiator cap and pressurize the system overnight....next day disable fuel and spark, and remove all plugs....now have someone crank over the engine and watch and see if coolant comes out of any of the holes......

In the stated scenario with only water in the cooling system during "really cold weather" the freeze plugs probably gave up the ghost, not the radiator.