  Millisecond pulsars in timing arrays can act as probes for gravitational wave
detection and improving the solar system ephemerides among several other
applications. However, the stability of the integrated pulse profiles can limit
the precision of the ephemeris parameters and in turn the applications derived
from it. It is thus crucial for the pulsars in the array to have stable
integrated pulse profiles. Here we present evidence for long-term profile
instability in PSR J1022+1001 which is currently included in the European and
Parkes pulsar timing arrays. We apply a new evaluation method to an expanded
data set ranging from the Effelsberg Pulsar Observing System back-end used in
the 1990s to that of data from the current PSRIX backend at the Effelsberg
Radio Telescope. We show that this intrinsic variability in the pulse shape
persists over time scales of years. We investigate if systematic instrumental
effects like polarisation calibration or signal propagation effects in the
interstellar medium causes the observed profile instability. We find that the
total variation cannot be fully accounted for by instrumental and propagation
effects. This suggests additional intrinsic effects as the origin for the
variation. We finally discuss several factors that could lead to the observed
behaviour and comment on the consequent implications.
