this is usually caused by some sort of damamge to the mother board during the repair process. if a component was knocked loose or knocked off then the phone would try to compinsate for the missing link. in return the phone works in over time mode and eventually shuts off either to cool itself down or to try t oreset itself to fix the problem.
9 times out of 10 its mother board damamge. you will just have to look really hard for the error. good luck and i hope this helps