2week old volvo owner. New to volvo so please be easy on me. I just got a 1999 s80 ( not t6) . From 40 to 60 at a set speed the car rocks front to back. The rpm's also bounce up n down 200rpm. Any ideas on what it is. U seem to know a hell of alot more then i can even try n pass off. Just as a kicker i also drive a volvo 18 wheeler gotta say good buy honda.

 

Hi and welcome to the forum. I have the same issue with my 850 sometimes. My 850 sedan did it all the time. My guess it that there is a loose vacuum connection or something.