Defective Xperia M5

I received my M5 Dual from Cell C about one month ago, and only managed to use it for 2 weeks when the device just shuts down even if battery is full. I have to then plug it into a charger before it will switch on again. I took the device back to Cell C, and just received it back after it was sent to the Sony Repair Centre. The phone was barely on for an hour and the issue occurred again. There is an issue with this phone, I have seen many other complaints of the same nature about the device. Extremely disappointed due to me having to pay a contract and insurance for a device I cannot use, and have to be inconvenienced by driving to Repair Centre to take device back.