  The linear stability of a shocked isothermal accretion flow onto a black hole
is investigated in the inviscid limit. The outer shock solution, which was
previously found to be stable with respect to axisymmetric perturbations, is,
however, generally unstable to non-axisymmetric ones. Eigenmodes and growth
rates are obtained by numerical integration of the linearized equations. The
mechanism of this instability is based on the cycle of acoustic waves between
their corotation radius and the shock. It is a form of the Papaloizou-Pringle
instability, modified by advection and the presence of the shock. As such it
can be generalized to non isothermal shocked accretion flows. Blobs and
vortices are generated by the shock as a by-product of the instability.
