  We analyze the slow periodicities identified in burst sequences from FRB
121102 and FRB 180916 with periods of about 16 and 160 d, respectively, while
also addressing the absence of any fast periodicity that might be associated
with the spin of an underlying compact object. Both phenomena can be accounted
for by a young, highly magnetized, precessing neutron star that emits beamed
radiation with significant imposed phase jitter. Sporadic narrow-beam emission
into an overall wide solid angle can account for the necessary phase jitter,
but the slow periodicities with 25 to 55 % duty cycles constrain beam
traversals to be significantly smaller. Instead, phase jitter may result from
variable emission altitudes that yield large retardation and aberration delays.
A detailed arrival-time analysis for triaxial precession includes wobble of the
radio beam and the likely larger, cyclical torque resulting from the changes in
the spin-magnetic moment angle. These effects will confound identification of
the fast periodicity in sparse data sets longer than about a quarter of a
precession cycle unless fitted for and removed as with orbital fitting.
Stochastic spin noise, likely to be much larger than in radio pulsars, may
hinder detection of any fast-periodicity in data spans longer than a few days.
These decoherence effects will dissipate as FRB sources age, so they may evolve
into objects with properties similar to Galactic magnetars.
