Repair of microwave

my mother has bought a LG Microwave serial no. 407TAYV7K906 at Makro Germiston when she got home and used it it made a funny noise and smelled like burning plastic she then took it back and they replaced it with the exact same brand and model. she used it and after using it for about 2 months it started doing the same as previous microwave. She then took it back to makro again on the 14-03-15 they gave her a number to contact LG and complain to them as well. when she did they responded and said there is nothing wrong with the microwave. why would we take it back to makro in the first place if apparently nothing is wrong with it. this is pathetic. this is the 2nd time we get an LG Microwave and both times it does the same it is still under warranty. we would never buy any product from LG again since if there is a problem with an electronic product they say there is nothing wrong. we will never recommend any products from LG to anyone and they don't even try to help you. my mother is very disappointed in them and their service. this is very bad and poor service!!!