I have a 2001 XJ8 with 39,000 miles and recently had two episodes where the transmission started shifting HARD. Both times it only lasted several minutes and then returned to normal. I had several messages pop up on the message center which were " gearbox fault", " TRAC not available" and something about the ACS. Also both times the ABS light came on and stayed on until things returned to normal.
The only common condition was that both time I had been driving slow for a few minutes (say 5-10 minutes at 15-20 MPH).
The second time this occured I stopped and shut the car off and then it would not start. I would only get a relay click and the starter would not turn at all. After sitting for a few minutes the car started and everything was fine. I have driven it approximately 150 miles since then without a problem.