Drop the coolant level down enough via the drain valve on the bottom of the radiator so you can see the top of the radiator core underneath the radiator cap. With a flashlight, look for telltale calcium deposits around the cooling channel openings indicating the radiator is becoming clogged with hard water deposits.
If you see mineral buildup inside, such deposits are caused by using tap water in the radiator instead of the required distilled water which is mineral free. At this point, I would suspect the radiator is getting clogged with those hardwater deposits. And while most radiators can be "rodded out" to get rid of such deposits, the TJ's radiator cannot be. Once it gets to that point, it can only be replaced because once it is taken apart to rod it out, it cannot be effectively resealed again since it is crimped together, not soldered.