By varying either the background pressure or the magnetic field strength at the cathode of an argon gas-fed hollow cathode arc, it is possible to suppress a low frequency coherent instability identified as an ion acoustic wave. An operating regime for this highly ionized plasma source is found where the rate of plasma diffusion across the magnetic field is classed as anomalous. This behaviour is associated with a large amplitude of the ion acoustic wave, but not with plasma turbulence or a drift wave. Electrode sheath conditions and the phase difference between the density and potential perturbations are shown to be more important in regard to the plasma losses than the steady state plasma parameters in the region of the plasma in which observations are made.