What causes s/d reading on a time grapher to jump all over the place?

I know the 6r35 is infamously unreliable and inaccurate. One I’m looking at right now jumps from +11s/d to -5s/d and everywhere in between when sitting on the timegrapher for 10 minutes. I mean every reading is different over that period. Amplitude is low (209 degrees or so). What causes the movement to be so inconsistent, or is it impossible to say without more information?