1998 Jaguar xj8 with 68,000 miles. All of a sudden my car starts decelerating all by itself while I am doing 70 mph on the by-pass, my accelerator won't co-operate and it scared me so I pulled over. The car is still running, but my accelerator is going down to the floor but not doing anything, suddenly it kicks in and a couple big puffs of smoke comes out of the tail pipes and it is fine and I am able to drive to work, what happened?