Transmission Noise problems of the 2015 Hyundai Genesis

While gradually accelerating from 20 through 40 miles an hour, the engine will rev to approximately 6000 rpm. During that hi rev, the car will not increase in speed (like it's in neutral) then after approximately 5 seconds, there is a loud bang noise in the car lurches forward and rapidly accelerates. It feels as if I were hit in the back by another car. This has happened twice in the 1 1/2 years that I have owned the car. Since it is a very intermittent and unpredictable problem. Even though I've brought the car into the dealer twice, they've been unable to find any defect. This is clearly a safety defect. I think, likely secondary to some transmission issue. The dealer and factory say there's nothing else they can do. Unless they have personally experienced the problem they will not do anything. I explained to them that even if they took the car on a six-month test drive, it might not happen at all. When it does happen, however, it is terrifying and dangerous.