hi
Ideally you want a logger on it to monitor all the sensors
on more modern Jaguar its quite easy and cheap to do, but dont know if there compatible with a 1992 car
Jaguar wds/ids should do it, would have to be a really early version, logging sensors then looking at the data should tell you whats causing it,
favorite items would be maf, ignition, timing or leaky injector
also have a look here http://www.jagrepair.com/ at the xjs technical service bulletins fault might be a common fault
cheers
Joe