Faulty 'new' device, and failure to replace device

I upgraded in December 2014, to a Sony Xperia Z3. The two weeks I had to evaluate the phone and give it back because of problems, went by and everything was fine. After that however, my phone started giving me lots of problems. It has been given in four times in the first eight months of my contract. I have requested a new device two times, without receiving a new one. I am a student and paying a lot of money for a phone which has been in repairs for a total of 3 months, totaling in me paying R 1500 while I could not use it. I have asked for my phone to be evaluated twice, but I do not know if they sent it through QC the second time. I cannot afford to pay for a phone which I can never use. This is very frustrating as I am paying for a 'new', but broken phone. It also costs me greatly in time and petrol every time I have to hand the device in and go and collect it again. I please request a new device, for the third time, as I feel as if I am being treated very unfairly. I think it is absolutely ridiculous and unacceptable that a 'new' phone be handed in for four times in the first eight months of a new contract. I have spoken to the head office, and was redirected once again.