We present the results of two-frequency flux density measurements of 74 pulsars with the Culgoora circular array. We show that the spectral index of a typical pulsar steepens markedly from 80 to 1400 MHz, but we found no significant relationship between the metre-wave spectral index and the published pulsar parameters.We investigated the intensity changes in 22 pulsars at 160 MHz over an l8-month interval and found that 19 of them were variable on a month-to-month time scale. A search for relationships between the degree of variability and the published pulsar parameters showed a highly significant dependence on dispersion measure and a probable dependence on period derivative. We attribute the dependence on dispersion measure to slow scintillations caused by large-scale turbulence in the interstellar medium, although the high amplitude of the intensity changes is not consistent with a recently proposed model for slow interstellar scintillation. The dependence of variability on period derivative and a high degree of correlation between intensity changes at 80 and 160 MHz demonstrate the presence of an intrinsic variability component.