By request about ten years ago, I posted pictures of broken solder joints in the body computer in a 1998 XJ8 and how to fix them. This could affect many different Jaguar models if they have computers.
The pictures have now vanished, so maybe it is worth a new post, since it took me about 3 years to find the fault after three mechanics and auto electricians failed to find the fault and charged a lot of money.

I was having random faults with the wipers and headlights and indicator lights. The headlights would go off for half a second every few minutes. The wipers would wipe randomly. The indicators would flash fast.
The reason for this is that the body computer (behind the glove box) has a huge cable with over 100 wires going to it, and the circuit board inside the computer is supposed to be earthed through two screws which have a big blob of solder on top. Over the years, that solder becomes cracked. So when there are lots of relays in use, the current is too much for the broken joints and the computer is no longer earthed. It takes half a second to reboot.
So, to troubleshoot this problem, with the engine not running, turn on the headlights and indicators and wriggle all the big cables (but mainly the cable under the glove box). If anything goes wrong, disconnect the battery and remove the body computer and solder up the two broken joints which will be very obvious. One day I will connect the circuit board to earth with a flexible wire, which is what should have been done in 1998. I gave photos of the broken joints to the local Jaguar dealers, who would usually sell you a new body computer to fix the problem.
I could explain how to remove the body computer if anyone wants. I don't suppose that is commonly done!

 

This happened to my 2000 xjr. I believe your post helped me repair it. Thanks again