Questions with Switching Gears and over reving at constant s

I've got a 2001 Jeep Cherokee 4x4 Sport with and Auto Trans. It's just when over 70K miles. I've just recently noticed that when driving at 50 or better it seems to jump between gears. When going 50mph it will jump from about 1500rpm to about 2100rpm. There is no change in modulation to the gas pedal and this is done on a flat road so there is no change in hill accent/decent. I first noticed the problem when the car was going between 60-70mph. The RPM's will jump up for a little bit then jump back down. It will continue to do this for a while. I'm not sure how long this would go on as my typical drives are not great in distance. It has been getting significantly colder and I am wondering if that could be part of the problem, but I fear that this isn't it. My inclination is that it might be: a sensor, something with the transmission, or throttle body (just putting it out there). Has anyone had a similar problem or knowledge of potential solutions to this problem?