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.
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