1998 Jeep TJ runs well then runs poorly

Hello

My jeep will run will for a while and then it will start to miss fire, it will back fire and will operate on a few cylinders. RPM will go up and down on its own. Can not stay still in park as idle will go up and down have to put car in neutral to stay still. Car will idle ok when not in gear but will stutter when load is put on the motor. Then, the problem will just stop and the car will run ok again. Usually the car starts and runs fine and then starts to do this after 5 to 15 min of operation. Is there anything I can do? Thanks

FWWR