The electron swarm parameter in a spatially uniform RF field has been investigated in Ar and in the model gas with the Ramsauer-Townsend minimum. Second-order analysis of the time dependent energy distribution has been executed over the range of angular frequency of the driving field between 10⁵ and 10⁸ s^-1 at an R. M. S. reduced-field of 50 Td and 1 Torr. Periodic nonlinear time behavior of the electron swarm has been exhibited and discussed from the viewpoint of the collisional relaxation time and the period of the driving field.