Originally Posted by Rebuilt
The only other things that I can really think of is if your radiator screen is plugged or matted down restricting airflow around the cooling veins. You mentioned that you flushed out the system a couple of times, but do you have tons of bugs and dirt in the cooling fins? Or if they have been damaged or matted down, you might be able to straighten them back out with a comb. If you have AC installed, the condenser may be blocking airflow through your radiator too. I don't know much about YJ's though. Have you eliminated the possibility of a bad temp gauge? Is your engine still quitting when it reaches temperature? (If so then this is probably not an issue) I would be curious and hook up an ohm meter to the terminals on the back of the gauge to measure resistance with the engine running and see if it changes as it registers hotter. If you see very little change in resistance but a large fluctuation in temperature reading, then it could be possible that your gauge is telling you stories. Some gauges actually have a placard on the back of them telling you the resistance range.
Sorry man, I know these are long shots. I'm just trying to throw more ideas out there. It sounds like you have pretty much done everything else.