We study a dynamic model of relaxor ferroelectrics based on the spherical random-bond---random-field model and the Langevin equations of motion. The solution to these equations is obtained in the long-time limit where the system reaches an equilibrium state in the presence of random local electric fields. The complex dynamic linear and third-order nonlinear susceptibilities χ_1(ω) and χ_3(ω), respectively, are calculated as functions of frequency and temperature. In analogy with the static

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... the dynamic model predicts a narrow frequency dependent peak in χ_3(T,ω), which mimics a transition into a glass-like state.