  We present a model-independent analysis of the short-timescale energy
dependence of low frequency quasi-periodic oscillations (QPOs) in the X-ray
flux of GRS 1915+105. The QPO frequency in this source has previously been
observed to depend on photon energy, with the frequency increasing with energy
for observations with a high ($\gtrsim 2$ Hz) QPO frequency, and decreasing
with energy for observations with a low ($\lesssim 2$ Hz) QPO frequency. As
this observed energy dependence is currently unexplained, we investigate if it
is intrinsic to the QPO mechanism by tracking phase lags on (sub)second
timescales. We find that the phase lag between two broad energy bands
systematically increases for $5$ - $10$ QPO cycles, after which the QPO becomes
decoherent, the phase lag resets and the pattern repeats. This shows that the
band with the higher QPO frequency is running away from the other band on short
timescales, providing strong evidence that the energy dependence of the QPO
frequency is intrinsic. We also find that the faster the QPO decoheres, the
faster the phase lag increases, suggesting that the intrinsic frequency
difference contributes to the decoherence of the QPO. We interpret our results
within a simple geometric QPO model, where different radii in the inner
accretion flow experience Lense-Thirring precession at different frequencies,
causing the decoherence of the oscillation. By varying the spectral shape of
the inner accretion flow as a function of radius, we are able to qualitatively
explain the energy-dependent behaviour of both QPO frequency and phase lag.
