  We report on the full analysis of a Rossi X-ray Timing Explorer (RXTE)
Proportional Counter Array (PCA) monitoring of the {\gamma}-ray binary system
LS I +61 303. The data set covers 42 contiguous cycles of the system orbital
motion. Analyzing this X-ray monitoring dataset, the largest to date for this
source, we report on the variability of the orbital profile and the spectral
distribution, and provide strong evidence for an anti-correlation between flux
and spectral index (the higher the flux, the harder the spectral index).
Furthermore, we present the analysis of two newly discovered ks-timescale
flares, which present significant variability also on shorter timescales, and
tend to occur at orbital phases between 0.6-0.9. However, a detailed timing
analysis of the flares does not show any coherent or quasi-coherent (QPO)
structure in their power spectra. We also investigated the possible appearance
of the radio super-orbital modulation at X-rays energies, but we could not
unambiguously detect such modulation in the system flux history, nor in the
evolution of its orbital modulation fraction.
