  We consider a $\pi$-mode solution of the Fermi-Pasta-Ulam $\beta$ system. By
perturbing it, we study the system as a function of the energy density from a
regime where the solution is stable to a regime, where is unstable, first
weakly and then strongly chaotic. We introduce, as indicator of stochasticity,
the ratio $\rho$ (when is defined) between the second and the first moment of a
given probability distribution. We will show numerically that the transition
between weak and strong chaos can be interpreted as the symmetry breaking of a
set of suitable dynamical variables. Moreover, we show that in the region of
weak chaos there is numerical evidence that the thermostatistic is governed by
the Tsallis distribution.
