I just bought a 1999 S80 with a 2.9L in it. Not the T6. From 40 to 60 mph when at a set speed the car rocks front to back and the rpm's bounce about 200. The car will still take off if u put ur foot in it theres no shaking, weired noises or anything. I tryed driving it on different types of blacktop and cement roads with the same outcome. Any ideas or has anyone else had this problem.