What would cause my car to raise and lower as I stop and go

maybe this,seems to be a common problem,costly to fix,around 6 Grand or more!......................... Tandem Hydraulic Pumps May Leak From Either Section on Mercedes-Benz CL500 Some models have tandem hydraulic pumps—one section for power steering and the other for suspension adjustment. These pumps are prone to leakage from either section. RepairPal Verified Average mileage: 68,178 (57,000–83,000) 5 model years affected: 1998, 1999, 2002, 2003, 2004