It is shown that under certain previously neglected conditions, not implausible and consistent with ptofit-maximisation, the Courtnot oligopoly solution is unstable : a saddle poinu Rn whose unstable manifold has dimension one, a line, and is contained in (Rn++URn__UO). That is, output-deviations from equilibrium are (or become) all pf the same sign : boom or collapse for the whole industry.