When she is experiencing the overheat, is she moving along at highway speeds (40+) or driving slowly, in traffic, through lights, etc...? Or both?
Overheating at speed (or both situations) is typically a coolant flow problem, meaning that a bad thermostat, bad waterpump, clogged cooling system, collapsing hose etc... is keeping enough coolant from getting through the radiator to cool the engine. Yes, a head gasket can fall in this category.
If it overheats at low speeds or idle, but NOT while cruising at speed, it's usually an airflow problem, I.E. the mechanical fan clutch, the E-fan or a damaged/missing fan shroud. Those don't matter above about 30-40 because the car moving is cramming more air over the rad than they could provide, but they are common culprits of low-speed/stopped/stop and go overheats.
It can of course be caused by multiple factors, and XJ cooling systems are sensitive to maintenance (or lack thereof) to begin with. It's worth going through Tim's entire list to be sure.
Lifted, Locked, Stiffened
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