  We present the results of a detailed analysis of RXTE observations of class
$\omega$ which show an unusual state transition between high-soft and low-soft
states in the microquasar GRS 1915+105. Out of about 600 pointed RXTE
observations, the source was found to exhibit such state transition only on 16
occasions. An examination of the RXTE/ASM data in conjunction with the pointed
observations reveals that these events appeared as a series of quasi-regular
dips in two stretches of long duration (about 20 days during each occasions)
when hard X-ray and radio flux were very low. The X-ray light curve and
color-color diagram of the source during these observations are found to be
different from any reported so far. The duration of these dips is found to be
of the order of a few tens of seconds with a repetition time of a few hundred
seconds. The transition between these dips and non-dips which differ in
intensity by a factor of ~ 3.5, is observed to be very fast (~ a few seconds).
It is observed that the low-frequency narrow QPOs are absent in the power
density spectrum (PDS) of the dip and non-dip regions of class $\omega$ and the
PDS is a power law in 0.1 - 10 Hz frequency range. There is a remarkable
similarity in the spectral and timing properties of the source during the dip
and non-dip regions in these set of observations. These properties of the
source are distinctly different from those seen in the observations of other
classes. This indicates that the basic accretion disk structure during both dip
and non-dip regions of class $\omega$ is similar, but differ only in intensity.
To explain these observations, we invoke a model in which the viscosity is very
close to critical viscosity and the shock wave is weak or absent.
