  We present a theoretical model for the alternating phase lags associated with
QPO fundamental and harmonic frequencies observed in some Galactic black-hole
candidates. We assume that the accretion flow exhibits a transition from an
outer cool, optically thick accretion disk to a hot, inner advection-dominated
accretion flow (ADAF), and that the QPOs are related to small-scale
oscillations of the accretion rate and the transition radius. We present an
analytical estimate of the expected phase lags at the fundamental and first
harmonic frequencies of the QPOs and perform detailed time-dependent
Monte-Carlo simualtions of the radiation transport in the oscillating ADAF /
cool disk system. We find that this model is well suited to reproduce
alternating phase lags between the fundamental and the first harmonic. It also
naturally explains the trend observed in GRS 1915+105 that, as the soft X-ray
luminosity increases, the QPO frequency increases and the phase lag associated
with the QPO fundamental frequency changes sign from positive to negative. The
relation between the disk temperature and the QPO frequency observed in GRS
1915+105 is consistent with a secular instability modulating the disk
evaporation at the transition radius.
