Defy worst Appliance and service delivery

I bought a Defy gas stove and gas Oven early in November last year. DGS601. Had a gas installer do gas installation early in December. The Oven doesn't operate at a stable temperature and overheats, and we cannot bake. I bought a top of the range one with convection fan and grill, it set me back 8K,however cannot use it at all. Defy came out 7 times already and cannot resolve the problem (there seems to be a big competancy issue) I am at my wits end. The worst is they don't even phone me to give feed back. l have been where l bought it at A&E hyperworld in Trichardt Mpumalanga, and they have to keep calling Defy out again and again. I feel also unsupported by the place l bought the stove. We are fed up now. Why can't they just replace the defective appliance. I feel like the warrabtee is not worth the paper its written on I paid 8K, and had to save all my hard earned cash to get ahead of load shedding, just to be stuck with this defective device. Please replace your appliance Defy and do the right thing. Here is the 7th job number as reference job no ***. 3 times your maintenance guy came without tools and tried to convince us that your oven is not supposed to bake. Defy model DGS601 gas oven and stove is the make and model