I’ve had my truck for almost 12 years and never seen this happen . What’ it started doing was normally my truck runs below 190 (see pic) at all times unless it’s dead of summer and I’m towing and climbing . Even when my rad was 10 years of oil build up ( before I cleaned it last summer) it still never bounced. Put in a Cummins thermostat 2 years ago also. What I noticed the other day empty on the upper mountain highway ( cooler) wS the temp gauge would walk up to a little past 190 , then back to the normal position . Even coasting down a long hill it was still climbing and then back down. Even my buddy started watching it commenting “ it’s getting warmer” . Last few days in town I been witnessing the same pattern . Topped up the overflow bottle and rad a few gauge but that hasn’t changed it. Looking for some ideas thanks.

 

I would say, depending on how fast the fluctuation is taking place would determine if its a glitching gauge problem or a thermostat problem. If its jumping around then I'd say it could be a gauge cluster problem. If you have a laser temperature tool then you can confirm at the thermostat neck if the gauge is telling the truth.
My first reaction would be to say that the thermostat is faulty. There's people who swear that the Cummins thermostat is the only one to get but I have a NAPA one and its been fine for a long time. Maybe you're in need of a coolant change/flushing and/or your coolant level is not what you think it is.

 

All I can say is my 01 that I got with 4,000 miles on it has done the same thing since I got it.