Bad service from Hi Fi Corporation

I bought a Sinotec 40" TV from Hi Fi Corp 3 years ago, which included a 5 year warranty. It developed horizontal black lines through the screen. I took it back to their Northgate branch for repairs. 21 days later I collected it supposedly fixed. Well it was fixed for 10 days, then the lines re-appeared. Back it went. 20 days later, I went to collect it, but in the shop it wouldn't switch on and it was returned with a different remote. I left it again for them to sort out. 7 days later, I once again went to collect It worked in the shop and has worked for 8 days since, but alas this morning the black lines are back. Each time I demanded a replacement TV but was told they have to fix it. I'm a 74 year old retired person who cannot afford to continually keep returning the TV and also be without any TV for weeks on end. I'm at my wits end, not knowing where to turn next. Would love some advice. Regards from a disgruntled Hi Fi customer.