Stability phenomena and time constants of instabilities in thermal plasma jets are essential in thermal plasma applications. We present observation of a thermal plasma jet generated by DC (direct-current) argon plasma torch by 8x15 matrix of fast photodiodes. The arc current was intentionally modulated in intervals 0.25 s long interposed by unmodulated gaps of the same length. Record of plasma radiation was analyzed by Fourier transform yielding spatial distribution of oscillations at selected frequency intervals. Results show that the oscillations present during unmodulated part in some frequency intervals are suppressed in the modulated part of the record. Suppression of oscillations depends on the location in the jet. It is strongest near nozzle orifice and weakest in the shear layer for frequencies lower than the modulation frequency. Suppressed higher frequencies show opposite spatial characteristics.