  We consider pair production and decay of fundamental unstable particles in
the framework of a modified perturbation theory (MPT) treating resonant
contributions in the sense of distributions. The cross-section of the process
is calculated within the NNLO of the MPT in a model that admits exact solution.
Universal massless-particles contributions are taken into consideration. A
comparison of the outcomes with the exact solution demonstrates excellent
convergence of the MPT series at the energies near and above the maximum of the
cross-section.
