Third time in for repairs!!

On 3 September I renewed my contract&got a Samsung J1. When I got home I set up the phone. That evening the phone turned off by itself&was unable to turn it back on with the on/off button so I had to remove the battery&then the phone turned on. It happened a few times that evening so I returned the phone to the Bayside branch where it was sent away for testing (OBF). When the phone came back it was found to have no faults. The day after I received the phone back I again inserted the sim&memory card&proceeded to use the phone, then within an hour again the phone turned itself off&again I was unable to turn the phone back on without removing the battery. Took the phone back again to Bayside&was told it would have to be sent away, they didn't have any waybills so I went to Canal Walk where the phone was sent away. Again came back with no faults yet after starting to use the phone it switches off yet again. Took it back to Canal Walk&again it goes away for repairs. Comes back&yet again the phone switches off again!!! So back to CW&again&the phone has now gone away for repairs!! I am now paying for a phone I don't even have or have used for more than an hour!!!!!!