  We analyze a set of three RXTE Target of Opportunity observations of the
Galactic microquasar GRS 1915+105, observed on April 2000, during a
multi-wavelength campaign.
  During the three observations, a strong, variable low frequency (2-9 Hz)
quasi periodic oscillation (hereafter QPO), often referred to as the ubiquitous
QPO, is detected together with its first harmonic. We study the spectral
properties of both features, and show that : 1) their frequency variations are
better correlated with the soft X-ray flux (2-5 keV), favoring thus the
location of the QPO in the accretion disk; 2) the QPO affects more the hard
X-rays, usually taken as the signature of an inverse compton scattering of the
soft photons in a corona; 3) the fundamental and its harmonic do not behave in
the same manner: the fundamental sees its power increase with the energy up to
40 keV, whereas the harmonic increases up to $\sim 10$ keV. The results
presented here could find an explanation in the context of the
Accretion-Ejection Instability, which could appear as a rotating spiral or hot
point located in the disk, between its innermost edge and the corotation
radius. The presence of the harmonic could then be a signature of the
non-linear behavior of the instability. The high-energy (>40 keV) decrease of
the fundamental would favor an interpretation where most or all of the
quasi-periodic modulation at high energies comes, not from the comptonized
corona as usually assumed, but from a hot point in the optically thick disk
