Sony Xperia M5

I bought my brand new Sony Xperia M5, last week Tuesday, on a 24 month contract . MTN only activated my sim on Friday. My phone keeps shutting down unexpectedly and won't turn on unless it is plugged into a charging point. The phone then switches back on and is ready for use. This happens even when my battery is fully charged. I am very disappointed in both Sony and MTN .This is happening more and more frequently. Yesterday it happened twice and today it has happened four times .I have factory data reset my phone twice .Nothing works. Im fed up with this phone. I'm on holiday now and it's useless having a 21.5 megapixels and the phone unexpectedly shuts down. I went to MTN in Somerset Mall this morning and they were not helpfull either. They don't have stock and told me the device must go to repairs. Unbelievable I have a phone for a week and it needs repairs. Very very unhappy with the service recieved.