  We show that old isolated neutron stars in groups and clusters of galaxies
experiencing a Quark-Nova phase (QN: an explosive transition to a quark star)
may be the source of FRBs. Each of the millions of fragments of the
ultra-relativistic QN ejecta provides a collisionless plasma for which the
ambient medium (galactic/halo, the intra-group/intra-cluster medium) acts as a
relativistic plasma beam. The Buneman and the Weibel instabilities,
successively induced by the beam in the fragment, generate particle bunching
and observed coherent emission at GHz frequency with a corresponding fluence in
the Jy ms range. The duration, frequency drift and the rate are in agreement
with observed properties of FRBs. Repeats (on timescales of minutes to months)
are due to seeing multiple fragments each beaming at a different direction and
coming in at different times. Single (non-repeating) FRBs, occur when only
emission from the primary fragment is within the detector's sensitivity. Key
properties of FRB 121102 (its years of activity) and of FRB 180916.J0158+65
(its 16 day period) are recovered. The spatial and temporal coincidence between
SGR 1935+2154 and FRB 200428 finds an explanation in our model. We give
testable predictions.
