Unable to solve a problem on its own product

I, purchased an ST3 brand new in 2014 . I would think this is their flagship car. The vehicle has gone in 3 times within the space of a week for an under boosting issue. Ford cannot fix the problem. every time I leave the dealership, i'm told that the master technician is happy with the way the vehicle is running. Whats worse is that I was told I will have to pay the cost of the diagnostics. I sent through a complaint on the website, no response. I called the customer relations team on ***, got a reference number and 3 days later, still no response... <br> The worst possible service ever