While on vacation in Lake Tahoe, I was driving the car down a fairly steep grade. I shifted the car into M and was between 2-3 to help with braking. The car surges and can't hold itself back going down the grade. It did it every time we went out. I have never had an issue like this going down a hill before. I am pretty concerned because it seems like it keeps slipping. I took the car into the dealership and they couldn't duplicate the problem because we don't have steep enough grades!! The issue remains unresolved but continues to occur when driving it up in the mountains.