Usual suspects :
Steering Damper faulty / worn
Front Suspension Bushes worn
Steering rod ends worn
Steering Drop Arm loose
Swivel Preload incorrect
Worn Steering Box
Wheel bearings loose
A useful check is to have a good crawl around and have a look for signs of play with someone wiggling the steering wheel back and forth. Once you have eliminated any play in the bits above, then you can look at the steering damper and preload.