Decelleration

This morning on my way to work my 1998 jag xj8 with 68,000 miles on it that has been fine all of a sudden starts decellerating all by itself while I am doing 70 mph on the by-pass, my accellerator won't co-operate, it scared me so I pull over, the car is still running, but my accelerator is going down to the floor but not doing anything, then all of a sudden it kicks in and a couple big puffs of smoke comes out of the tail pipes and it is fine and gets me to work after all, what happened, it scared me.
thanks
Karen Bruner 8)