Your car uses a slave cylinder and separate control valve for the power steering. First look at that control valve to see if something is loose. When you turn the steering wheel, a passage in that valve opens to let pressurized fluid push on the slave cylinder. When the cylinder moves the steering linkage, that pulls the control valve closed and the movement stops unless you turn the steering wheel some more. That valve is not closing when you stop turning the steering wheel. That can be due to a sloppy linkage or a problem inside the valve assembly. Kits are available to rebuild those control valves.