  Multiplicity of phase states within frequency locked bands in periodically
forced oscillatory systems may give rise to front structures separating states
with different phases. A new front instability is found within bands where
$\omega_{forcing}/\omega_{system}=2n$ ($n>1$). Stationary fronts shifting the
oscillation phase by $\pi$ lose stability below a critical forcing strength and
decompose into $n$ traveling fronts each shifting the phase by $\pi/n$. The
instability designates a transition from stationary two-phase patterns to
traveling $n$-phase patterns.
