  We present a study of the flux and spectral variability of the two broad-line
radio galaxies (BLRGs) 3C 390.3 and 3C 120, observed almost daily with RXTE for
nearly two months each in 1996 and 1997, respectively. Our original motivation
for this study was to search for systematic differences between BLRGs and their
radio-quiet counterparts, the Seyfert galaxies, whose temporal and spectral
behavior is better studied. We find that both 3C 390.3 and 3C 120 are highly
variable, but in a different way, and quantify this difference by means of a
structure function analysis. 3C 390.3 is significantly more variable than 3C
120, despite its jet larger inclination angle, implying either that the X-ray
variability is not dominated by the jet or that two different variability
processes are simultaneously at work in 3C 390.3. We performed an
energy-selected and time-resolved analysis based on the fractional variability
amplitude and found that the variability amplitude of both objects is strongly
anticorrelated with the energy. This last result, along with the correlated
change of the photon index with the X-ray continuum flux, can be qualitatively
explained within the scenario of thermal Comptonization, generally invoked for
radio-quiet active galaxies. Moreover, the time-resolved and energy-selected
fractional variability analyses show a trend opposite to that observed in
jet-dominated AGN (blazars), suggesting only a minor contribution of the jet to
the X-ray properties of BLRGs. Time-averaged spectral analysis indicates the
presence of a strong, resolved iron line with centroid at 6.4 keV and a weak
reflection component in both objects. The overall PCA+HEXTE spectra are best
fitted with the constant density ionization model of Ross & Fabian, but with a
modest ionization parameter(abridged).
