periodically my 03 xjr will go into fail safe mode wher engin e revs drop to idle and provides just enough power to get off the road. This happens ahighway speeds as well as sitting idling or any other driving function. Most times it can be corrected by shutting off the ignition for approx 30 seconds and then restarting, once in a while a hard boot of the battery is required.Hav had the car into Jag specialists to no avail. There are NO codes showing when this happens but have noticed warning messages of no asc or traction control.Any ideas would be greatly appreciated.
Thanks Terry.

 

Weird that it doesn't show codes, but it's a typical indication of a Throttle Position Sensor issue, or also common to be the Throttle Body itself. Mine does the exact same thing, when dropping off the throttle (sometimes it goes days with no issues, some days it'll do it non stop). In my case, I replaced the sensor, no change. Next step is the TB but that'll be after my road trip. The fact that the specialists have found nothing, is a little puzzling though.

 

There's 2 sensors on the throttle body that Charlz has mentioned, the throttle position sensor or TPS and the throttle pedal position sensor or TPPS. As the car is drive-by-wire, either of these can trigger a failsafe mode (idle chug to the side of the road).
P1121 OR P1122.

Your symptoms are a classic sign on either sensor tracks are worn, with a momentary signal drop leading to failsafe. Many on the forum have had their TB's rebuilt because of these worn parts - if you do a search on the forum you'll find the company that offer a refurb which is waaay cheaper than a new TB.