On my '00 it has been two things.
The CPS which almost left me stranded on the side of the road, 1/2 mile from home though, which I believe was experiencing thermal failure. Finally got her started and made it home. Replaced it right then.
The starter. Leaving work one day would not start. Gave it a love tap and started right up. Happened again 2 days later and I just replaced it before it stranded me for real. I think because of the position of the starter under the oil filter that when previously owned it just got doused in oil every time the oil was changed and caused it to start acting up finally. I put rags on top of it when I do the oil change.

The oil on the starter problem of XJs has pros and cons. Oil doesn't get "in" the starter as much as it gets all over it. If the connections are clean and tight, the oil can actually act as a protectant for the connections and outer casing and keep corrosion from making them fail. So for those in the rust belt (like me) this can be a little helpful. But if the connections are not clean and tight, the oil can get between them and sometimes compromise the connection which can of course lead to starter failures. I clean mine off when I detail the motor, but other than that I leave it a lone now.