Hey guys, new to the forum so bear with me, I will try to describe the issue i'm having as best i can.

So i just bought a 99 XJ with 111k miles on it. It runs great and idles perfectly at 750 rpm. Today after i changed the oil i wanted to warm up the oil to make sure the level was correct so I had the jeep in Park and held the throttle steady at 2000 rpm. It was holding steady at 2000 and then jumped up to about 2500-2800 ish for a few seconds and then fell back to 2000. I thought it might be a fluke so i turned the car off, started it again and held it steady at 2k rpm. It held steady for at 2k for about 10 seconds then jumped to 2500 for about 3 seconds and then fell back down to 2k and continued this fluctuation until i turned the car off again. Any ideas what would cause this? It drives fine and doesnt surge when im driving around town. Any suggestions would be great.