I was just now driving my 98 XJ for about 15 minutes and the temp gauge went from about 220 all the way to the red and check gauge came on. I pull over and lift the hood and notice my coolant reservoir tank was 3/4 full but the radiator cap was not very hot. Today was the first day that i have had a reading over 210.

Just a little background I had just got my oil changed and two oil leaks fixed, one at valve cover gasket and the other oil filter adapter. I was planning on flushing the coolant this weekend due to brown gunk in my coolant.

Can anyone help me with why it jumped like that and why it went back to running around 220 afterward?

 

Buy a new radiator cap

Its dumping into your overflow.

 

^^I second that^^
Also get your system flash as soon as you can. Also when you do the flush change out you t-stat, Check out you water pump and flush your radiator and heater core separately from the engine. Let us know once you flush the system if it help or not.