general problems of the 1998 Jaguar XJ8

Vehicle recently purchased used from auto broker. Since purchase, there has been a vibration seemingly originating from the right front wheel area. During an exceptionally windy day (30+ knot crosswinds on highway), the vibration was strong enough to make me question my ability to control the vehicle. I assumed this was due to tires out of round/balance or vehicle out of alignment so purchased 4 new tires and had vehicle aligned. Vibration is still noticeable although not nearly as strong as before. Still notice that when strong gusty conditions are in the area that the vibration increases accordingly. Possibly related condition is that when sunroof is open and vibration occurs I can hear a strumming sound like there is a problem with the aerodynamics of the vehicle.

While driving 50 mph + vehicle vibrates which may possibly cause a loss of vehicle control. Dealer cannot determine cause. Please provide further information.