Faulty Handset

My phone was booked in now for the 3rd or 4th time to Mtn repair centre midrand each time it was booked in they sent the phone to Sony (sss) they said it was software & each time it came out the problem just got worst each time! I picked up my on Friday (20/03/2015) I was told that a software upgrade was done after getting home starting it up not even 20min after switching it on it overheats to an extent which it cannot be held & screen freezers really tired of such pathetic service! It is very hard to believe the phone has passed Qa. When the phone heats up all the time To an extent it cannot be held. When the phone was booked in the lady at the front desk felt the phone and her colleague asked why is this phone so hot. When returning after it was sent to Sony a simple software upgrade was done. How does a phone malfunction in this way and they say it's a software issue