You could check the actual temp with a hand held temp meter, the gauges are notorious. However, a sudden change in reading normally means something is afoot.
Flushing the system, running with water, flushing again and refilling with proper coolant could solve your problems, but unlikely. I am trying a summer coolant additive this year, probably a waste of time. You could try a rad flush but the citrates in most of them are fairly mild and may not have a great effect. You could take the rad to a rad specialist for a service.
You are dealing with an ageing system that was marginal when new, the engine bay is very crowded. If there are no leaks and obvious signs of failed parts, it is probably falling performance.
If your pump is working and your radiator is not blocked (???) the problem is often that the fan is not working hard enough. The mechanical fan is the workhorse in an XJ and has a thermostatic fan clutch. The fan clutch wears with time and the amount of power it transfers to the fan when hot starts to drop and the fan blades start to slow. From the factory it may have had about 30% slip, over time this increases and after 10 years it may be slipping 60% plus. If you have an original fan clutch, it may be time to replace it. They are available in various duty ranges, a "heavy duty" clutch merely slips less, using more engine power to turn the fan faster.
If you are fighting high ambient temperatures, look at one that slips very little. This will increase fuel consumption!!!
BagusJeep lives in Bali.
1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE