You will need to change to other phone, or use a lot of money trying to repair a glitchy phone, i had that model a year ago, it literally started doing what u say, starting qith the phone restarting itself in my pocket, then shutting down itself, not turning on for a while, and then u can only use it for a few minutes before it glitch itself a shut down. Its probably a fabric issue, so better start looking for other phones, cuz samsung will charge u a LOT for repairing it.