I have just purchased a 2003 Cadillac CTS with 23,000 miles on it. We test drove it on smooth roads but when we purchased it and drove it home on the interstate with payment sections we feel EVERY bump in every payment section. The dealer changed tires, (Goodyear) to Michilin and that made some difference but it's stll very bumpy. Could this be the shocks? What are your suggestions that we might try.