  Experiments on the $Gd^{3+}$ Electron Spin Resonance (ESR) in the filled
skutterudite $Ce_{1-x}Gd_{x}Fe_{4}P_{12}$ ($x \approx 0.001$), at temperatures
where the host resistivity manifests a smooth insulator-metal crossover,
provides evidence of the underlying Kondo physics associated with this system.
At low temperatures (below $T \approx K$), $Ce_{1-x}Gd_{x}Fe_{4}P_{12}$ behaves
as a Kondo-insulator with a relatively large hybridization gap, and the
$Gd^{3+}$ ESR spectra displays a fine structure with lorentzian line shape,
typical of insulating media. The electronic gap is attributed to the large
hybridization present in the coherent regime of a Kondo lattice, when Ce
4f-electrons cooperate with band properties at half-filling. Mean-field
calculations suggest that the electron-phonon interaction is fundamental at
explaining the strong 4f-electron hybridization in this filled skutterudite.
The resulting electronic structure is strongly temperature dependent, and at
about $T^{*} \approx 160 K$ the system undergoes an insulator-to-metal
transition induced by the withdrawal of 4f-electrons from the Fermi volume, the
system becoming metallic and non-magnetic. The $Gd^{3+}$ ESR fine structure
coalesces into a single dysonian resonance, as in metals. Still, our
simulations suggest that exchange-narrowing via the usual Korringa mechanism,
alone, is not capable of describing the thermal behavior of the ESR spectra in
the entire temperature region ($4.2$ - $300$ K). We propose that temperature
activated fluctuating-valence of the Ce ions is the missing ingredient that,
added to the usual exchange-narrowing mechanism, fully describes this unique
temperature dependence of the $Gd^{3+}$ ESR fine structure observed in
$Ce_{1-x}Gd_{x}Fe_{4}P_{12}$.
