phone keep on over heating and text is also damage

My Samsung S4 Mini was in for repairs at Samsung SA, when I got it back the problem is still not fixed its still over heating and now the text on the phone is also starting to be damaged if you type a message it looks like there is a line through it. if was handed in at Vodacom 4 times and every time they say there is nothing wrong. I have a contract the phone is still under warranty im paying for a phone that I could not have used for the last 6 month this is pathetic I WILL NEVER EVER BUY A SAMSUNG AGAIN. I INSIST ON A NEW PONE BECAUSE THERE WAS ENOUGH ASSESSMENTS DONE ON THIS THING CALLED A PHONE AND I STILL HAVE 9 MONTHS LEFT BEFORE MY CONTRACT ENDS. SAMSUNG HAVE ALREADY REPLACED THE MAIN BOARD AND VODACOM DID SOFTWARE UPDATES. SO NOW I WANT A NEW PHONE PLEASE. I CAN NOT KEEP ON PAYING AND PAYING FOR A PHONE THAT BEING ASSESSED AND REPLACED EVERY OTHER WEEK.