Kia Cerato 2015: Mystery problem causing revs to drop?

Earlier this year we purchased a new Kia Cerato Si with the 2.0-litre engine. When you start it and the revs start to drop it shakes violently. My wife took the car to the dealer where it was purchased, but they weren’t able to diagnose the problem. We spoke to Kia Australia who claimed it would be the alternator pulley, which the dealership then changed the pulley, but the problem was still apparent. Due to poor customer service we decided to take it to a different dealer, and they had the car three days but were unable to work out what was wrong. They said it’s possibly the fuel and to try high octane for two fills. I'm going to try this, but I cannot see how it’s going to make a difference. Can you shed some light on the problem, or tell me which way to turn, as it’s becoming extremely frustrating.