I have a 2000 (W reg) XJ 4.0 V8 Sovereign lwb. I frequently get the warning messages come up saying "ASC not available" & "Trac not available" then it goes int restricted performance mode. If I stop and turn the engine off, 9 times out of ten when I turn the ignition back on the fault has cleared. It might then run for a couple of days or a couple of minutes, before it happens again.
Anyone have any ideas? ;)