Hi Andy,
Firstly is the tracking set up correctly?
It's also worth checking that the 2 bolts that attach the rack to the car haven't worked loose. It will only take a small amount of loosening to make the car wander horrendously. I had this happen to me recently. - All was fine for 2 or 3 months after changing a rack, then one day it started wandering dangerously almost as if driving on ice.
Hope this helps.
Matt
September 2008