  This paper presents the stability analysis of a system sliding at low
velocities ($< 100 \mu$m.s$^{-1}$) under a periodically modulated normal load,
preserving interfacial contact. Experiments clearly evidence that normal
vibrations generally stabilize the system against stick-slip oscillations, at
least for a modulation frequency much larger than the stick-slip one. The
mechanical model of Bureau {\it et al.} (2000), validated on the steady-state
response of the system, is used to map its stability diagram. The model takes
explicitly into account the finite shear stiffness of the load-bearing
asperities, in addition to a classical state- and rate-dependent friction
force. The numerical results are in excellent quantitative agreement with the
experimental data obtained from a multicontact frictional system between glassy
polymer materials. Simulations at larger amplitude of modulation (typically 20%
of the mean normal load) suggest that the non-linear coupling between normal
and sliding motion could have a destabilizing effect in restricted regions of
the parameter space.
