  The bright pulsar PSR B0329+54 was previously known for many years to have
two emission modes. Sensitive observations of individual pulses reveal that the
central component of pulse profile, which is called core component, is found to
be very weakened occasionally for some periods and then recovered. This is the
newly identified core-weak mode. Based on our long observations of PSR B0329+54
by the Jiamusi 66-m telescope at 2250 MHz, we report here that the profile
components of individual pulses, including these for the core and the leading
and trailing peaks, are relatedly varying over some periods even before and
after the core-weak mode, forming a regular pattern in the phase-vs-time plot
for a train of period-folded pulses. The pattern has a similar structure for
the core-weak mode with a time scale of 3 to 14 periods. It starts with an
intensity brightening at the trailing phase of the core component, and then the
core intensity declines to a very low level, as if the core component is
drifting out from the normal radiation window within one or two periods. Then
the intensity for the trailing components is enhanced, and then the leading
component appears at an advanced phase. Such a core-weak mode lasts for several
periods. Finally, the core-weak mode ends up with an enhanced intensity at the
leading phase for the core component, as if the core gradually comes back and
finally stays at the phase of the profile center as it used to.
