1998 Jaguar XJ8 V8 Two Wheel Drive Automatic 121,000 miles. We just finished installing spark plugs, and ran a diagnostic check, after the car would not start. It was started and stopped within a few minutes and would not restart. We had to purge all the gas out of cylinders and the new plugs. Now when I'm driving and engage the cruise control the control turns off and I get a message, FAILSAFE ENGINE MODE? Hope you have some ideas.