  In this paper we pursue the origin of the non-Gaussianity determined by a
bispectrum analysis of the COBE-DMR 4-year sky maps. The robustness of the
statistic is demonstrated by the rebinning of the data into 12 coordinate
systems. By computing the bispectrum statistic as a function of various data
partitions - by channel, frequency, and time interval, we show that the
observed non-Gaussian signal is driven by the 53 GHz data. This frequency
dependence strongly rejects the hypothesis that the signal is cosmological in
origin. A jack-knife analysis of the coadded 53 and 90 GHz sky maps reveals
those sky pixels to which the bispectrum statistic is particularly sensitive.
We find that by removing data from the 53 GHz sky maps for periods of time
during which a known systematic effect perturbs the 31 GHz channels, the
amplitudes of the bispectrum coefficients become completely consistent with
that expected for a Gaussian sky. We conclude that the non-Gaussian signal
detected by the normalised bispectrum statistic in the publicly available DMR
sky maps is due to a systematic artifact. The impact of removing the affected
data on estimates of the normalisation of simple models of cosmological
anisotropy is negligible.
