  The phenomenon of subpulse drifting may hold the key to understanding the
pulsar emission mechanism. Here, we report on new observations of PSR
J0034-0721 (B0031-07), carried out with the Murchison Widefield Array at 185
MHz. We observe three distinct drift modes whose "vertical" drift band
separations ($P_3$) and relative abundances are consistent with previous
studies at similar and higher frequencies. The driftbands, however, are
observed to change their slopes over the course of individual drift modes,
which can be interpreted as a continuously changing drift rate. The implied
acceleration of the intrinsic carousel rotation cannot easily be explained by
plasma models based on ExB drift. Furthermore, we find that methods of
classifying the drift modes by means of $P_3$ measurements can sometimes
produce erroneous identifications in the presence of a changing drift rate. The
"horizontal" separation between driftbands ($P_2$) is found to be larger at
later rotation phases within the pulse window, which is inconsistent with the
established effects of retardation, aberration, and the motion of the visible
point. Longer observations spanning at least ~10,000 pulses are required to
determine how the carousel rotation parameters change from one drift sequence
to the next.
