A plane going up and down 20-30 feet seems like it would be very unpleasant. Considering that there's longitude and latitude, isn't it more likely that the altitude is coming from GPS, which is notoriously inaccurate with regards to elevation?
GPS altitude is used for vertical guidance for certain types of GPS approaches (i.e. "LPV" approaches[1]) and requires the airplane's avionics to be equipped with a WAAS[2] receiver that provides accurate altitude information.
[1] https://en.wikipedia.org/wiki/Localizer_performance_with_ver...
[2] https://en.wikipedia.org/wiki/Wide_Area_Augmentation_System
At cruise altitude, you're moving along at 500 mph, which is 777 feet per second. So going from +30 feet to -30 feet in a minute is just an adjustment of only about 5 degrees. You'd barely feel it, even walking down the isle. An acceleration of 33 ft/sec per sec is 1 g.
You experience greater changes in vertical motion on any flight you go on.
*edit: units
You would pretty obviously feel a change in pitch of 5° walking down the aisle.
You mixed feet per second and feet per minute. 60 feet of change across 777 feet of run is about 4.5° (inverse sin(60/777)), such as you'd experience if the change was in 1 second instead of in 1 minute.
Calculating 60' change in 777*60 feet, inverse sin (60/(777*60)) is 0.07°, which is why you don't feel that change in inclination of the aisle.
(Although personally, I agree with the sibling comment that the variability is likely an artifact of the sensor resolution.)
btw: Aren't you the guy who tracks planes flying in circles? I follow you on Twitter. Such a cool project!