  Access to 50 years of data has led to the discovery of pulsar emission and
rotation variability on timescales of months and years. Most of this long-term
variability has been seen in long-period pulsars, with relatively little focus
on recycled millisecond pulsars. We have analyzed a 38-pulsar sub-set of the 45
millisecond pulsars in the NANOGrav 11-year data set, in order to review their
pulse profile stability. The most variability, on any timescale, is seen in
PSRs J1713+0747, B1937+21 and J2145-0750. The strongest evidence for
long-timescale pulse profile changes is seen in PSRs B1937+21 and J1643-1224.
We have focused our analyses on these four pulsars in an attempt to elucidate
the causes of their profile variability. Effects of scintillation seem to be
responsible for the profile modifications of PSR J2145-0750. We see evidence
that imperfect polarization calibration contributes to the profile variability
of PSRs J1713+0747 and B1937+21, along with radio frequency interference around
2 GHz, but find that propagation effects also have an influence. The changes
seen in PSR J1643-1224 have been reported previously, yet elude explanation
beyond their astrophysical nature. Regardless of cause, unmodeled pulse profile
changes are detrimental to the accuracy of pulsar timing and must be
incorporated into the timing models where possible.
