Cell is shuting off after about 3 mins of talking on it!

It is a sprint cell phone its a sanyo,I just took it to a sprint store and they said it needed updated so they updated it and they said everything else was fine that nothing was wrong with the cell.They said my battery was fine,but I did not tell them that I have dropped it a few times but it just started this about 2 months ago now it was out of the blue,the screen where it tells you the time on the outside she messed up.They said that wouldn't cause the phone shutting off and on,its only a 3 month phone it wasn't a brand new cell it was used but everything was fine with it,it will keep shutting off till it either messes up and freezes or you take the battery out. Please help if Anyone can please!!!!!!!!!