Sounds like you might have some play in the rack. You can confirm this by carrying out the checks that Steve suggests above. If you do confirm that it is play in the rack then have a look at this tip that came from someone else here (sorry forgot to write down who was responsible for this gem)
'You can adjust out slight play in a steering rack as long as its not completely knackered, Look where the steering coloumn meets the rack and you'll see on the rack a plate with two securing screws and a screw in the middle, twist the screw in the middle in till you feel it starting to bind then wind it back very slightly.'