Same thing happening to me on 2000 Jag xk8. Also sometimes the engine light symbol also comes on. Am told it is a faulty electronic throttle control assembly that has several sensors with high failure rates, or it is the connector pins (if silver, need to be gold plated). New assembly costs $2100 plus labor, so someone needs to confirm for sure before you go through with it. Usually fault codes P0121/0507.