Mine is a a 2000 XJR with over 120K on the odometer. I had posted a thread many month back when this problem reappeared after two full years of happy motoring. Several trips to my Jag exclusive mechanic had born no fruit as to the cause and once, even had to have the car towed to him.
He could not duplicate the problem. In it's latest iteration, the car would not start reliability once up to operating temperature, but would start after cooling off. Several more trips to another mechanic yielded no positive results or diagnosis. Then I talked to the wife. She'd been chatting with her friends and mentioned the problem, to which she got the advice to check the battery cables; her friend had had a problem in her Mercedes of the same kind.
I dismissed the advice as too simple to be worthwhile, but yet, when I took the Jag to the mechanic for the last visit, I mentioned the suggestion and sure enough, he wiggled the battery cable and was able to replicate the problem. No start. Wiggle. Started! He ordered a new OEM battery cable for $120 (!) and the problem was solved. I've driven from Utah to California and back with no starting issues and so far, the car is dead stock reliable. Moral of the story is, sometimes the simplest solutions are the best and always listen to the wife.