  We present a simple and generic theoretical description of actin-based
motility, where polymerization of filaments maintains propulsion. The dynamics
is driven by polymerization kinetics at the filaments' free ends, crosslinking
of the actin network, attachment and detachment of filaments to the obstacle
interfaces and entropic forces. We show that spontaneous oscillations in the
velocity emerge in a broad range of parameter values, and compare our findings
with experiments.
