  GRS 1915+105 was observed by BeppoSAX for about 10 days in October 2000. For
about 80% of the time, the source was in the variability class $\rho$,
characterised by a series of recurrent bursts. We describe the results of the
timing analysis performed on the MECS (1.6--10 keV) and PDS (15--100 keV) data.
The X-ray count rate from \grss showed an increasing trend with different
characteristics in the various energy bands. Fourier and wavelet analyses
detect a variation in the recurrence time of the bursts, from 45--50 s to about
75 s, which appear well correlated with the count rate. From the power
distribution of peaks in Fourier periodograms and wavelet spectra, we
distinguished between the {\it regular} and {\it irregular} variability modes
of the $\rho$ class, which are related to variations in the count rate in the
3--10 keV range. We identified two components in the burst structure: the slow
leading trail, and the pulse, superimposed on a rather stable level. We found
that the change in the recurrence time of the regular mode is caused by the
slow leading trails, while the duration of the pulse phase remains far more
stable. The evolution in the mean count rates shows that the time behaviour of
both the leading trail and the baseline level are very similar to those
observed in the 1.6--3 and 15--100 keV ranges, while that of the pulse follows
the peak number. These differences in the time behaviour and count rates at
different energies indicate that the process responsible for the pulses must
produce the strongest emission between 3 and 10 keV, while that associated with
both the leading trail and the baseline dominates at lower and higher energies
