  We present 20 years of timing observations for 1E 1207.4-5209, the central
compact object in supernova remnant PKS 1209-51/52, to follow up on our
detection of an unexpected timing glitch in its spin-down. Using new XMM-Newton
and NICER observations of 1E 1207.4-5209, we now find that the phase ephemeris
can be well modelled by either two small glitches, or extreme timing noise. The
implied magnitudes of the frequency glitches are Delta f/f = (9+\-2)E-10 and
Delta f/f = (3.7+/-0.7)E-10, at epochs 2010.9 and 2014.4, respectively. The
updated timing solutions also rule out our previous suggestion of a large
glitch in the frequency derivative fdot. No other canonical pulsar with such a
small spin-down rate (fdot = -1.2E-16 Hz/s) or surface dipole magnetic field
strength (B_s = 9.8E10 G) has been observed to glitch; the glitch activity
parameter of 1E 1207.4-5209 is larger than that of more energetic pulsars.
Alternative parameterizations that do not involve glitches can fit the data,
but they have timing residuals or a second frequency derivative fddot that are
orders of magnitude larger than in pulsars with similar spin-down parameters.
These timing properties of 1E 1207.4-5209 further motivate the leading theory
of central compact objects, that an initial B-field of normal strength was
buried in the neutron star crust by fallback of supernova ejecta, suppressing
the surface dipole field. The slow reemergence of the buried field may be
involved in triggering glitches or excess timing noise.
