Pathetic servicing by Samsung

I currently have a Galaxy S4 that I have under contract from July 2013. Recently I came across a problem where my phone kept on restarting and I booked it in to the Repair Centre in Nandi Drive 3 weeks ago. As soon as I got the phone back in a weeks time, another problem was found where the Wi-Fi signal on my phone was absolutely non existent at any distance. So once again I took it back for this problem, I then found out that my screen had been changed for no apparent reason whatsoever because I took it back with my screen fully functional. Also one of my ADH chances had been used which I feel did not need to happen. Then I picked my phone up once again on this week and without fail another problem occurs where the external speaker stopped working. I took it back yesterday demanding that my phone be replaced but to no avail. My phone is still under warranty and the only reason I demanded a new phone was because the \world class engineers"don't have any idea what they are doing. Fix one problem and cause another. I definitely don't trust Samsung with my business anymore. You expect a certain high standard and I never experienced that. """