There have been numerous posts about erratic heart rate tracking on the Fitbit models with a heart rate monitor, and equally as many posts recommending adjusting the band location and/or tightness. Today I was on a long sweaty run and noticed at about the 6.75 mile point my Fitbit Surge was moving around on my arm; apparently I had forgotten to notch the band down one hole prior to my run.
I made a note to myself to check the heart rate track and look to see if there was any obvious change in tracking consistency before and after the 6.75 mile mark. Sure enough, my heart rate plot smooths out precisely where I tightened the band down one notch; see the following image: