  We derive and employ a semi-classical Langevin equation obtained from
path-integrals to describe the ionic dynamics of a molecular junction in the
presence of electrical current. The electronic environment serves as an
effective non-equilibrium bath. The bath results in random forces describing
Joule heating,current-induced forces including the non-conservative wind
force,dissipative frictional forces, and an effective Lorentz-like force due to
the Berry phase of the non-equilibrium electrons. Using a generic two-level
molecular model, we highlight the importance of both current-induced forces and
Joule heating for the stability of the system. We compare the impact of the
different forces, and the wide-band approximation for the electronic structure
on our result. We examine the current-induced instabilities (excitation of
runaway "waterwheel" modes) and investigate the signature of these in the Raman
signals.
