Over heating when sitting at idle for a short while

leave the radiator cap off while its idling untill it reaches the temp of the termostat that will open and the air will be pushed to the radiator

Yes, cause that sensor operates the electric cooling fans by the radiator, if those fans or fan don't come on, the radiator can't cool down as quick and causes the engine to run hotter.

you electric fan probably isnt working with vehicle warmed up lift hood and watch temperature gauge when it starts to over heat check to see that fan is working if not it could be the fan relay or fan itself if the fan does turn on its most likley the thermostat

Replace the thermostat?

This might help you as it looks as though you have an electric fan to cool , there should be a sensor on your radiator hoses upper and lower.they are actually switches triggered by the coolant temperature in side the hose. While your Jeep is sitting idle, shut off the AC as it adds to the heat off the engine not to merntion useing more gas. you should make sure both high and low speeds on the coolant fan work. good luck if this does'nt help check out the post, with more, on the 1995 Jeep heating up..

Many cars have a thermoswitch threaded into the radiator that activates the fans when engine exceedes normal optimum opperating temp. This sensor may have failed , or one of the 2 wires that go to it may have come unpluged or broken

follow top radiator hose from radiator to engine wear hose ends, it will be clamped to whats called the gooseneck, thermostat is under that. you will have to drain coolant, not all, maybe 1 half gal. before you remove it. 2 bolts new gasget new thermostat, just remember spring on new thermostat goes in engine.

If it's lack of heat, it could be the heater control valve. If the jeep has alot of miles on it, the heater core should be reverse flushed.

The problem with these jeeps is that the older they get the more clogged the radiator geats.
The radiator in the Cherokees are only a single row of tubes.
You can try to flush out the radiator, but replacing it with a heavy duty type is probably a better option.
Your car is not getting enough flow to cool down the engine.
Also you want to check that the electric fan is coming on. There is a coolant sensor in the radiator.

It's behind the passenger headlight.Need to take off grill,bumper fascia to get access to it.Mine took about an hour alltogether.