There are several issues here that can cause your problem...
1) Wear in the steering rack and / or track rod ends
2) Worn out shock absorbers (they play a huge role in keeping the vehicle stable)
3) Worn strut top bearings - hidden on the top of the spring, they allow the spring to turn with the strut.
Poor quality tyres or incorrect tyre pressures can play a part too!
John
August 2009