  We analyze theoretically the collective mode dispersion in multi-layer stacks
of two dimensional dipolar condensates and find a strong enhancement of the
roton instability. We discuss the interplay between the dynamical instability
and roton softening for moving condensates. We use our results to analyze the
decoherence rate of Bloch oscillations for systems in which the s-wave
scattering length is tuned close to zero using Feshbach resonance. Our results
are in qualitative agreement with recent experiments of Fattori {\it et al.} on
$^{39}$K atoms.
