To start my XJ is a 2000 Jeep Cherokee Sport 4.0 and everything under the hood is stock and everything having to do with the radiator is original and any help is appreciated. Went for a drive yesterday, after about 40 min of highway driving with no issue (staying at about 210 degrees) got off the exit and check gauges light kicked on and temp spike to about 250ish (the tick before 260) and after a couple mins it just up to 260. Popped the hood and coolant was dripping out of the overflow of the coolant reservoir. I did have one other coolant issue about a year ago when the coolant sludged up in the reservoir blocking the intake causing overflow (dumped it, topped it off and has worked fine since). Assumed this was the same issue so I dumped the reservoir again topped it off. It then no longer overfilled the reservoir but it continued to over heat and now sprayed/ leaked coolant at the cap. I replaced the cap today as the seal was getting a bit old anyway. Started it again today and let run for about 30 min. Same as before, overheating. A few additional things I noticed. First is doesn't steadily heat up, instead after gradually heats up past 210 and to around 220ish then needle stops for a few mins until jumping immediately to 250ish with the "check gauges" light on. Also during today's test, the reservoir didn't seem to be filling (was at the "full" line when i started) but after overheating the reservoir had significantly more full, Almost like there may be a clog and suddenly forced itself out but as it was still overheating. Next after reading some other forums from people with similar but not quite the same issue, a few people were mentioning possible issues that involved the AC and the condenser. I will state not the AC has not worked in years (I live in Maine its not a huge issue) so I have no idea about any of those components working or not. The one exception to that is the electric fan, which even though its for the AC, I am told it does help cool the system, well either way it is working, kicks on when the vehicle hits about 210. If i left out any important/ pertinent information just let me know

So what I'm looking for- Any ideas on what the issue could be and how to diagnose to make sure.
If there is no easy way to diagnose, what order should I start replacing parts to find whats broken (or clogged)

thanks ahead