Kia Sorento 2015: Transmission problem

My husband and I bought a 2015 Kia Sorento Platinum in June. At about 60km/h and 80km/h the engine is misfiring, shunting, surging. The dealership has yet to find a solution. It occurs when you ease your foot off the accelerator and then gently increase the speed. It happens often when driving but is more prevalent when the engine is cold. We initially thought it was the engine then changed our minds that it was the transmission. As it appears this is an issue with more than just our car it would be appreciated if you could advise if you have a solution as to how this can be fixed.