I have had my 99 A6 for almost 2 years now. Just recently the Check Engine Light was on so I brought it in for that and some recalls. The dealership replaced the Throttle Valve (recall) and said that probably caused the Check Engine Light issue. They were wrong so I brought it back and they replaced the Air Mass Flow Sensor and Throttle Valve (the first replacement was defective) and said that should do it. Shortly after that the Check Engine Light came back on and the car started running very rough at certain speeds (55-65mph for example). So I brought it back in and had a bunch of needed service performed: Two new rear tires, alignment, timing belt, water pump, oil change. They determined that the Check Engine Light came on because the Air Mass Flow Sensor they installed was defective (2nd defective component installed in two trips from a dealership). I got it back the other day and the car still isn't running as well as when I brought it in. It is not a smooth ride, there is some studdering or rough ride between 55-65mph, and the idle doesn't always seem right. I plan on calling the dealership, but I was curious if there was any help out there first. There is also an issue when I first start the car after leaving work. It seems to stick before changing gear once I leave my parking spot. Someone suggested using 89 Octane, I currently use 91 Octane, but I don't notice this in just the cold weather. Thanks for any help that might be out there.

 

My car used to shudder at high speeds also, it was caused by a faulty O2 sensor.