We consider the problem of the long-time stability of plane waves under nonlinear perturbations of linear Klein-Gordon equations. This problem reduces to studying the distribution of the mode energies along solutions of one-dimensional semilinear Klein-Gordon equations with periodic boundary conditions when the initial data are small and concentrated in one Fourier mode. It is shown that for all except finitely many values of the mass parameter, the energy remains essentially localized in the initial Fourier mode over time scales that are much longer than predicted by standard perturbation theory. The mode energies decay geometrically with the mode number with a rate that is proportional to the total energy. The result is proved using modulated Fourier expansions in time.