XJ's tend to have issues overheating. XJs changed from Closed to Open Cooling systems in 1990.
Here are some troubleshooting steps to determine the cause.
Check for Air in the system
Sometimes if you overheat, have a bad radiator cap or general leak, you can get air in the system. The easiest way to eliminate all air is to have a Tefba Coolant filter mounted at the highest point of the radiator hose. The Tefba Coolant filter provides an easy entry to fill the cooling system and burps air since its at the highest point.