Rpms fluctuate 300 to 400 rpms when driving

Could be a lot of things - no way to diagnose from that single symptom.

I’m happy to help further over the phone at https://www.6ya.com/expert/donald_f2ed37026a3ac881

Have you checked the level and the color of the transmission fluid on the transmission fluid dip stick?

low fuel pressure from air / low fuel level and dirty filters may be the fault.

My Jeep Grand Cheroke Limited 2000 did the same thing and the dealer replaced the O2 sensor and problem was solved. It about drove me crazy until it was figured out.

Is the check engine light on ? How does it run now that it has cooled off ?

Standard Shift? Clutch is Slipping

Automatic? It is coming out of gear or slipping

If it is an automatic-- you most likely will end
up having to rebuild it,if it is original since built for yr 2000

Problem with electronic throttle control maybe. Any light's lit on the instrument cluster or messages engine power reduced . Could be a sign a position sensor is going bad . The gas pedal assembly has to sensors as does the throttle body . You could try cleaning the throttle body bore . Having a qualified repair shop hook up a diagnostic scan tool to view sensor data would be a good thing .
007 Idle Speed Problems Scan Data for Idle
1 reason your cars throttle body needs cleaning Gain lost horsepower and...

Yes..... The TPS can cause this!!! Another sign of a failing TPS is for the rpm's to fluctuate up and down by 100-300 as you drive down the interstate at a steady speed on a level surface.

If it is automatic transmission with overdrive the lock-up torque converter is kicking in and out causing the rpm to fluctuate.Could be just the converter or maybe the overdrive going out.

Were you towing in tow haul mode?