The stability of traveling waves is investigated for a model describin
g the post threshold behavior beyond oscillatory instabilities for a c
lass of systems with a conserved order parameter. Oscillatory instabil
ities and their post threshold behavior in systems with unconserved or
der parameters have been a central topic of nonlinear science during t
he recent decade. The most famous equation in this context is the Ginz
burg-Landau equation with complex coefficients. Here I discuss a strai
ght forward generalization of this Ginzburg-Landau equation which cove
rs also spinodal decomposition and the crossover to an oscillatory ins
tability for a globally conserved order parameter. Especially, the mod
ification of the border to spatiotemporal chaos is considered, which i
s described by the so-called Benjamin-Feir resonance.