I would look at the exhaust ports between 3&4 under the valve cover for a crack. I do have a 2000 XJ. It has 260K on it. I don't know if the head has been replaced. I've had it only 2 years or so. If it is loosing coolant through the head, your oil should be milky. You should be able to see the area between the 3&4th exhaust ports through the oil cap hole. That is usually were they crack. Could be a warped head.
A few alternate ideas. First is the thermostat. If it is bad, ie malfunctioning, the temps might be low in cold weather. Another is the water pump. I have had an impellor come lose on a Dodge Neon. The motor would be fine, then spike, then get cool, then spike again. I'd also replace the radiator cap. The seal could be bad, or the spring weak. Usually, replace a radiator cap every 5 years. Its an item I replace on any used car I buy.
I did have a '78 Gremlin. It loosed coolant. Guys said to replace the thermostat. I did. Still, lost coolant. Then I read "Auto Repair for Dummies' by Dena Sclair. In that, the radiator cap regulates the pressure of the coolant system. Keeps it from boiling.
Do not dismiss the cracked or warped head idea. If it is that, you can damage the engine. I did get an engine that was damaged for that issue, sans the head. I sold a TUPY head to the dealership from an '03 WJ. The cam was wiped out. Later that block was used to get a TJ running from parts from 3 motors.
