  The stability of an expanding parton plasma is analyzed within quasi-particle
models. The effective mass of the parton is calculated self-consistently from a
gap equation which is either obtained from the Nambu Jona-Lasinio Lagrangian or
from a fit to observables calculated on the lattice. The latter shows an
effective confinement. At thermal equilibrium the stability is studied within
thermodynamics (mechanical stability) and via a linear response analysis of the
Vlasov equation. The instabilities related to a first-order phase transition
are found. For a plasma expanding in three and one dimensions far from
equilibrium a new type of instability, called dynamical, appears. The relation
to cluster formation is shown in a molecular dynamics calculation.
