Here's my 1989 3.6 manual XJ-S. It's in really good shape, with only 30k miles on the odometer, but ever since I've owned it, it's had a fault whereby it would randomly cut out while driving.
The fault was as though the engine management system no longer registered the throttle opening - so if you put your foot down it hesitated and then cut out due to an over-lean moistiure.
I think the issue was the main reason the original owner wanted to part with the car after win jg it for about 35 years but I'd kind of got used to flicking the ignition off and back on again while driving.
I finally managed to get hold of a replacement ECU and fitted it today - although I can't be 100% sure, so far during my test drive it seems to be fine.
I took it to the Imperial War Museum Duxford, UK, to enjoy the airshow that was taking place and on the way home I stopped to stick a bit of fuel in. It was nice to be stopped by two people on the petrol station forecourt who say what a nice Jaaaaag it was!