Why does the car feel like it's twitchy at high speed?

have you had your wheels balanced?

Most XK8's have a common problem with the suspension bushings. The rubber they are made of is only good for about 5 years or 60,000 miles. After that the bushings get soft. The worst culprit is the upper shock bushing on the front suspension. Right now my car needs new upper bushings, but they are $150 to $250 each and the labor will run you twice that much. A couple of companies make kits to rebuild the bushing assemblies. That saves a little money, but it is still labor intensive.