I`d also second the fix the leak suggestion..Even though it may be a small leak you are losing pressure as well as any fluids.

 

my friend i will give you some advice from our little experience ,my friends XJ overheated ,he changed the fan in the water pump,and he gets good results,maybe you have to change your thermostat.In my car ,i put the ventilator full hot,when OVHT ,and drops the temp. real quick,if you have a sudden problem.Wish you best of luck . GREECE always expect you to explored

 

If you read at the top I just put a brand new thermostat in.

 

I used JB weld for a pin hole in my CJ's radiator and it worked great so far. But thats a quick fix for a cheap radiator.