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Multidirectional optical diagnostics combined with fast Fourier transform analysis yielding frequency spectra were used for investigations of oscillation phenomena in a thermal plasma jet on various frequencies. The results show that oscillation amplitude distributions in the plane perpendicular to the jet axis depend on the frequency value and, for some frequencies, are strongly asymmetrical. This phenomenon affects the mean distribution of instabilities in a different way, depending on the frequency range. Instantaneous instability distributions are influenced by the phase of arc-current low-frequency modulation.