Poor vehicle diagnosis and failure to rectify the problem but charge an arm and a leg

I took my Polo TSI 2017 to NTT VW East London for overheating when driving uphill. They deduced the diagnosis to be an Axillary water pump and was replaced at R8 500.00. I drove the car a few days and took i long distace and it did the same thing. I reported to my service advisor and adviced me to return the car. I did today and she told me after it had been in the workshop that they don't know what the cause could be. I feel ****** by NTT VW having to take my car to them as professionals and pay R8 500.00 yet the problem persists, and they say they don't know what is the matter. So how can they advice us to buy cars from them when they can't even fix them? They however take monies under pretext of fixing them.