On the way to a camping trip, highway driving into the foothills, my xj's temperature, which has been around 220, goes to max. I pull over, shut it down, wait about 2 min, then start back up. Temp quickly drops back down to 220, while I watch. Drove another 5 miles, then same thing. Happened about 5-6 times, total. Not promising for a weekend off road, right? So, we turned around, drove home, back down the highway about 3 hours. Temp issues didn't happen again.

Background: The radiator is topped up with fresh coolant, but hasn't had a flush in years. However, coolant color doesn't look bad, and no sludge or buildup I can see. Fan comes on when the engine is hot at idle. Jeep has an aux oil cooler that was installed for towing.

Any ideas what's going on? If it was just an overheating issue, wouldn't the temp rise gradually, then decrease gradually after shutting the engine off? Why would the temp spike and drop super-quickly?

A radiator flush is one thing. However, I would look more at a thermostat problem.

Also, check the cooling fins on the radiator. Often the fins are aluminum and susceptible to salt air corroision.

On the same theme, have you run through mud lately? If so, the fins on the radiator may be clogged with mud.