  We study the dynamics of small fluctuations about the uniform state of a
crystal moving through a dissipative medium, e.g. a sedimenting colloidal
crystal or a moving flux lattice, using a set of continuum equations for the
displacement fields, and a one-dimensional driven lattice-gas model for the
coupled concentration and tilt fields. For the colloidal crystal we predict a
continuous nonequilibrium phase transition to a clumped state above a critical
Peclet number.
