Bought a 2002 xj8 convertible last summer. Sweet ride, but there is some front end jitter that won't go away. Dealer recommended new upper steering arm. Replaced…that didn't work.Front shocks replaced…made it better but still jittery. The entire front end has been checked and rechecked by dealer and three good mechanics and they find nothing wrong. all components are tight. Tires balanced and alignment done. there is constant vibration in steering wheel and if you hit a pothole the wheel really lets you feel it! Any ideas??