  We analysed 9 RXTE/PCA observations of GRS 1915+105 in the flaring state,
when the hardness of the source spectrum was changing on the time scales from
few seconds up to 1000 seconds. The quasi periodic oscillations (QPOs) with the
frequency varying between ~ 1 and 12 Hz are associated with the episodes of
harder source spectrum. In each observation we found tight correlation between
the duration of the hard episode and the characteristic QPO frequency. For a
half of the observations this correlation matches the relation between the
viscous time scale and the Keplerian frequency, when both quantities are
evaluated for various radii in the radiation pressure dominated accretion disk.
Assuming that the QPO frequency is proportional to the Keplerian frequency at
the boundary between an optically thick accretion disk and a hot comptonization
region, the changes of the QPO frequency can then be understood as due to
variations of this boundary position on the viscous time scales.
