I had overheating issues plague both my XJ's at some point in time. I tried a number of fixes but inevitably the cure was to ensure EVERY component in the cooling system was healthy. The last issue I'd had was a new radiator cap from Supercheap wasn't doing it's job, replaced it with one from Bursons...problem fixed.
So even the most minor of components can screw you over!
You shouldn't need to go to extremes to keep the XJ cool, just the basics.
One incident that caused me some grief was after replacing a water pump the XJ started running hot, it had not run hot prior but the pump needed to be changed because it developed a leak.
I had another near new radiator handy, swapped it in and problem solved. The diagnosis was that the increased flow from the new water pump must have dislodged some crap in the block and clogged the existing (very healthy) radiator...Grrr!
In summary, what I'm saying is don't try fancy mod's to fix the problem, just go over all the factory components systematically and you will solve the over heating.
Good luck
