  Aims. To investigate the flux density modulation from pulsars and the
existence of specific behaviour of modulation index versus frequency. Methods.
Several pulsars have been observed with the Effelsberg radio telescope at 8.35
GHz. Their flux density time series have been corrected for interstellar
scintillation effects. Results. We present the measurement of modulation
indices for 8 pulsars. We confirm the presence of a critical frequency at ~1
GHz for these pulsars (including 3 new ones from this study). We derived
intrinsic modulation indices for the resulting flux density time series. Our
data analysis revealed strong single pulses detected from 5 pulsars.
