A cooling system has a few challenges. Water works good to remove heat. But it freezes. Enter antifreeze. Problem solved.
Water boils at 212 degrees. Enter the radiator cap. The higher the pressure, the higher the boiling point. Your cooling system should be under pressure when hot. A radiator cap typically holds 15 pounds of pressure, which translates into a higher boiling point.... around 45 degrees higher, or 255 degrees. When it exceeds that pressure, it blows it out into the overflow tank.
Whats that mean? It is either very very hot, or the radiator cap is not holding pressure......
Check your cap next, since you already changed the stat.
I'm not feeling confident this is going to have a good ending........
Good luck, let us know what you find.