Does the sub frame move? Never heard of them need changing, but wouldnt rule them out. If it is the case i would contact Powerflex and see what they can offer, you may have hit on something everyone else has missed.
Could be any joint in the steering and front suspension.
As above i have replaced everything except ball joints (?), top mounts (ok) and track rod ends and i stil get a knock on speed bumps. Unless theres something loose.
Jack car up and disconnect a drop link and give the wheel a good tug in all directions, should make it easier to trace the play in other areas. If no joy, drop links and roll bar bushes is left, neither last that long.