  PSR J1825$-$0935 (PSR B1822$-$09) switches between radio-quiet (Q-mode) and
radio-bright (B-mode) modes. The Q-mode is known to have a periodic fluctuation
that modulates both the interpulse and the main pulse with the same period.
Earlier investigators argued that the periodic Q-mode modulation is associated
with drifting subpulses. We report on single-pulse observations of PSR
J1825$-$0935 that were made using the Parkes 64-m radio telescope with a
central frequency of 1369 MHz. The high-sensitivity observations revealed that
the periodic Q-mode modulation is in fact a periodic longitude-stationary
intensity modulation occurring in the interpulse and the main pulse. The
fluctuation spectral analysis showed that the modulation period is about $43
P_1$, where $P_1$ is the rotation period of the pulsar. Furthermore, we confirm
that the modulation patterns in the interpulse and the main pulse are
phase-locked. Specifically, the intensities of the interpulse and the
immediately following main pulse are more highly correlated than for the main
pulse and interpulse at any other lag. Polarization properties of the strong
and weak Q-mode states are different, even for the trailing part of the main
pulse which does not show the periodic intensity modulation.
