  We study the relaxation towards thermodynamical equilibrium of a 1-D
gravitational system. This OSC model shows a series of critical energies
$E_{cn}$ where new equilibria appear and we focus on the homogeneous ($n=0$),
one-peak ($n=\pm 1$) and two-peak ($n=2$) states. Using numerical simulations
we investigate the relaxation to the stable equilibrium $n=\pm 1$ of this
$N-$body system starting from initial conditions defined by equilibria $n=0$
and $n=2$. We find that in a fashion similar to other long-range systems the
relaxation involves a fast violent relaxation phase followed by a slow
collisional phase as the system goes through a series of quasi-stationary
states. Moreover, in cases where this slow second stage leads to a dynamically
unstable configuration (two peaks with a high mass ratio) it is followed by a
new sequence ``violent relaxation/slow collisional relaxation''. We obtain an
analytical estimate of the relaxation time $t_{2\to \pm 1}$ through the mean
escape time of a particle from its potential well in a bistable system. We find
that the diffusion and dissipation coefficients satisfy Einstein's relation and
that the relaxation time scales as $N e^{1/T}$ at low temperature, in agreement
with numerical simulations.
