The power spectrum of the temporal fluctuations in the occupation number of an ionic shell has been evaluated using a model Hamiltonian that represents configurations hybridized with conduction-electron states. Explicit forms for the autocorrelation function and the power spectrum are derived. The results of M\"ossbauer and x-ray photoemission spectra experiments in intermediate-valence systems are analyzed and the connection with Kubo's stochastic theory of line shape is given. The conclusions shed light on the validity of the concept of temporal fluctuations when applied to intermediate-valence systems.