Unsatisfactory Servicing

Hi, i recently purchased a Sony Experia Z3 from your store in N1 City. After a month of having the phone it started doing wierd things ie. apps were opening and closing, without me touching the phone. Certain actions coud not be done ie. unable to click 'ok' or 'delete' if i needed to. I took the phone in for repiars and received it back last week Friday 31 July. It has passed all the relevant tests while being at the service centre however when i got my phone back it just did the same thing. This phone is only two months old and i am paying my contract faithfully every month but I am sitting with a product that is not functioning as it was intended to function. Infact it is malfunctioning. This is very frustrating and it seems as if all that Vodacom can do for me is to send it in for repairs again. That does not make any sense beacuse if they did not know what was wrong with the phone and hence could not fix the issue the first time how are they able to do it the second time around. I am demanding a new phone because I am within my right as a customer to get what I pay for. I will revert to the Ombudsman with my matter if i am not satisfied.