  We present X-ray observations of the Narrow-Line Seyfert 1 galaxy PKS
0558-504 obtained with RXTE during a 1-year monitoring campaign. This source,
which is one of the very few radio-loud NLS1 galaxies, shows strong X-ray flux
variability on time scales of weeks-months accompanied by spectral variability.
The main goal of this study is to investigate the spectral variability with
model-independent methods and time-resolved spectroscopy in order to shed light
on the origin of the X-rays. The main results can be summarized as follows: 1)
The flux typically changes by a factor of 1.5-2 on time scales of 10-30 days,
with few extreme events where the flux increases by a factor of ~4 in 3 days.
2) We do not observe any large amplitude, flux related spectral variations.
During the flux variations, the spectrum varies mainly in normalization and not
in shape. We do observe some small amplitude spectral variations, which do not
correlate with flux, although there is a hint of spectral hardening as the
source brightens. 3) There is no evidence for reprocessing features such as the
Fe Kalpha line or a Compton hump. We argue that PKS 0558-504 is a peculiar
object that appears to be different from most of the radio-quiet and radio-loud
AGN. If a jet is responsible for the bulk of the X-rays, it must operate in an
unusual way. If instead a corona is responsible for the X-rays, the system
might be a large-scale analog of the Galactic black holes in the transient
intermediate state.
