Likely to be the thermostat. Radiator and water pump are other possiblities, but the thermostat is less expensive and easy to change, so I'd do that first. The fan clutch is another possibility. Under what conditions does it run hot?

__________________
Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.