The structural changes of ferroelectric Rb(2)ZnCl(4) have been studied in real time under the influence of pulsed electric fields by high-resolution gamma-ray diffraction close to the lock-in transition at 198.3 K. A well-defined field-induced switching between the incommensurate and the commensurate phase is observed only in virgin crystals on a time-scale of milliseconds. Crystals aged under high electric fields, however, exhibit an intermediate diffuse phase that is attributed to pinning effects of discommensurations, i.e. the walls of nano-domains in the incommensurate phase. Surprisingly, the incommensurate phase itself is stabilized and the transition into the ferroelectric phase is shifted to lower temperatures as soon as the frequency of the cycled electric field is increased. This unusual finding is discussed in terms of stress fields associated with the motion of discommensurations.