Steering

I have a 2001 Mazda Millenia. It is a 2.3L V6 with approx 70000 miles on it. We are having problems with the steering in certain situations. When turning the steering wheel there is a repetitive vibration that occurs. It almost feels as though the something is slipping. After a few seconds it stops, but if I turn the wheel the other directin it will occur again for a few seconds. It only occurs when we are stopped or moving vey slowly (for instance when we are parallel parking. Any ideas?