Wandering/vibration

HI My 1995 300 TDI disco keeps wandering allover the road and has a vibration from the steering. I have changed the wheel bearings,swivell bearings and tightend them down. Its got gas shocks and heavy duty springs and a gas steering damper. The a frame ball joint was relplaced and theres no play in it. All bushes have been replaced with poly bushes. The steering shaft has been replaced and had a new heavy duty universal joint. I swapped the steering box for another one but it made no differance. Any ideas as to whats causing it?

Check the panhard rod is done up properly. It has to be torqued to 130Nm.
If its not tight you will see the bolts moving about when someone moves the steering wheel.

cheers i will check the pan hard rod and let you kmow how i got on