Why buy the phone in the first place, if they are all suspect?
P.S. What do you mean by overheating? Do you mean the phone get warm, or so hot is shuts off completely, burns you, makes the screen change colors, melts plastic? Overheat has been used with alot of drama here so the term is not what it means anymore... If it is the former, they all get warm. If it is the later, then you have a bad phone.
If you are talking and charging at the same time, try using the USB port from a computer instead of the charger that comes with the phone. Maybe a lower current charge might cause less heat etc...