Unhappy Mercedes Benz Constantia Kloof customer

My name is Nelson, I'm a driver of a Mercedes Benz c63 AMG performance pack 2012 model which I bought in 2015 and I fully enjoyed the vehicle. Upon enjoying that car in 2017 late I came across the newer c63s which I fell in love with and bought in 2018. In July 2018 my car lost power as I was driving from kzn to Johannesburg and luckily it was during the day and I called Mercedes to come and assist which they did and took me car to Bethlehem dealership to have it checked and yes we got there and in no time my car was sorted and I was ready to go . They just changed the spark plug. I drove back to Johannesburg and I was still not happy with the performance and I decided to take the car to constantia kloof Mercedes ro get it checked . From then the rest is history , I drove the car with their workshop manager on numerous occasions and he said there was nothing wrong with my car. Clearly he was sure what he was talking about because to date my car is still at their workshop as I told them I wont take until they checked what was the problem . They then came back to me telling me there was something wrong with one of the cylinders which was very funny as their workshop manager kept saying my car was fine all these months. I find him very *********** . He doesn't even have the decency to say yes Nelaon you were right and I was wrong all along . Now my worry is one their customer service , two is that cars maintenance plan is going to end soon and I didn't even get to enjoy my car because it literally spent the whole time with them . And I'm spending thousands a month on that car . I really loved the brand but that's the last one I was ever buying due to their service especially the workshop manager. He even had a nerve to say I drive the car hard as if I was driving a Toyota Corolla 1.3 or something of that sort. They made me to hate the brand