  We present an analysis of approximately 13-yr of observations of the
intermittent pulsar B1931+24 to further elucidate its behaviour. We find that
while the source exhibits a wide range of nulling (~4-39 d) and radio-emitting
(~1-19 d) timescales, it cycles between its different emission phases over an
average timescale of approximately 38 d, which is remarkably stable over many
years. On average, the neutron star is found to be radio emitting for 26 +- 6 %
of the time. No evidence is obtained to suggest that the pulsar undergoes any
systematic, intrinsic variations in pulse intensity during the radio-emitting
phases. In addition, we find no evidence for any correlation between the length
of consecutive emission phases. An analysis of the rotational behaviour of the
source shows that it consistently assumes the same spin-down rates, i.e. nudot
= -16 +- 1 x 10^-15 s^-2 when emitting and nudot = -10.8 +- 0.4 x 10^-15 s^-2
when not emitting, over the entire observation span. Coupled with the stable
switching timescale, this implies that the pulsar retains a high degree of
magnetospheric memory, and stability, in spite of comparatively rapid (~ms)
dynamical plasma timescales. While this provides further evidence to suggest
that the behaviour of the neutron star is governed by magnetospheric-state
switching, the underlying trigger mechanism remains illusive. This should be
elucidated by future surveys with next generation telescopes such as LOFAR,
MeerKAT and the SKA, which should detect similar sources and provide more clues
to how their radio emission is regulated.
