  A generic model for alternating lags in QPO harmonics is presented where
variations in the photon spectrum are caused by oscillations in two parameters
that characterize the spectrum. It is further assumed that variations in one of
the parameters is linearly driven by variations in the other after a time delay
$t_d$. It is shown that alternating lags will be observed for a range of $t_d$
values. A phenomenological model based on this generic one is developed which
can explain the amplitude and phase lag variation with energy of the
fundamental and the next three harmonics of the 67 mHz QPO observed in GRS
1915+105. The phenomenological model also predicts the variation of the
Bicoherence phase with energy, which can be checked by further analysis of the
observational data.
