  We report the results of a systematic timing analysis of RXTE observations of
GRS 1915+105 when the source was in its variability class theta, characterized
by alternating soft and hard states on a time scale of a few hundred seconds.
The aim was to examine the high-frequency part of the power spectrum in order
to confirm the hecto-Hertz Quasi-Periodic Oscillations (QPO) previously
reported from observations from mixed variability behaviours. During the hard
intervals (corresponding to state C in the classification of Belloni et al.,
2000, A&A, 35, 271), we find a significant QPO at a frequency of ~170 Hz,
although much broader (Q~2) than previously reported. No other significant peak
is observed at frequencies >30 Hz. A time-resolved spectral analysis of
selected observations shows that the hard intervals from class theta show a
stronger and steeper (Gamma=2.8-3.0) power-law component than hard intervals
from other classes. We discuss these results in the framework of hecto-Hertz
QPOs reported from GRS 1915+105 and other black-hole binaries.
