Ford Falcon: Wobbling steering wheel

I have a 2004 BA series Falcon XR6 which has done just over 95,000 km it has developed a quite violent steering wobble when cruising over 80 km/h. A specialist we went to replaced the brake rotors, performed a wheel alignment and balance, and did a comprehensive check of all the steering components, but it didn't resolve the underlying problem of steering wobble. We took it back to our local Ford dealer to have it checked and they said it was fatigue in the brake booster causing it to lock on the brakes at a certain atmospheric pressure. It has since been replaced but the problem still persists. I am at a loss as to the reason why, please help.