  We report on collective non-linear dynamics in an optical lattice formed
inside a high finesse ring cavity in a so far unexplored regime, where the
light shift per photon times the number of trapped atoms exceeds the cavity
resonance linewidth. We observe bistability and self-induced squeezing
oscillations resulting from the retro-action of the atoms upon the optical
potential wells. We can well understand most of our observations within a
simplified model assuming adiabaticity of the atomic motion. Non-adiabatic
aspects of the atomic motion are reproduced by solving the complete system of
coupled non-linear equations of motion for hundred atoms.
