  We apply the Green function formalism for $t-\bar t$ production and decay
near threshold in a study of the effects due to the momentum dependent width
for such a system. We point out that these effects are likely to be much
smaller than expected from the reduction of the available phase space. The
Lippmann--Schwinger equation for the QCD chromostatic potential is solved
numerically for $S$ partial wave. We compare the results on the total cross
section, top quark intrinsic momentum distributions and on the energy spectra
of $W$ bosons from top quark decays with those obtained for the constant width.
