Hi, I have a problem with my much loved E200 automatic estate car. I have found that it will sometimes not hold on a hill in gear but at other times it will hold as normal. It also seems very strange when it changes gear, sometimes it changes normally but other times it does not change down at all and I need to "kick down" to make it change. I sometimes hear a kind of "pop" like a backfire when it changes gear, does anyone know what this means? I would be so grateful for any help in this.