According to posts on the internet, this is a very common problem and a dangerous one with Jags. The car will go into "limp mode" and won't go any faster than 25 miles an hour and may not make it uphill. If this happens where there is no place to go to the side of the road, then traffic will be blocked and may cause accidents. Jaguar apparently does not acknowledge the problem. The throttle body assembly needs to be replaced.