Linear and nonlinear wave propagation in optically thin, impurity seeded radiative plasmas is studied in slab geometry. Taking into account the thermal force that acts on impurity ions leads to a new type of slightly damped or slightly unstable subsonic waves that propagate within limited ranges of temperature. The one-dimensional nonlinear equation reduces to that of a classical particle in a forced external field with nonlinear friction. It is shown that the slightly unstable wave may saturate due to the nonlinear character of the ``friction'' term, and the stability of the nonlinear solution is analyzed.