  By means of neutron scattering we show that the high-temperature precursor to
the hidden order state of the heavy fermion superconductor URu$_{2}$Si$_{2}$
exhibits heavily damped incommensurate paramagnons whose strong energy
dispersion is very similar to that of the long-lived longitudinal f-spin
excitations that appear below T$_{0}$. Since the underlying local f-exchange is
preserved we expect only the f-d interactions to change across the phase
transition and to cause the paramagnetic damping. The damping exhibits
single-ion behavior independent of wave vector and vanishes below the hidden
order transition. We suggest that this arises from a transition from valence
fluctuations to a hybridized f-d state below T$_{0}$. Here we present evidence
that the itinerant excitations, like those in chromium, are due to Fermi
surface nesting of hole and electron pockets so that the hidden order phase
likely originates from a Fermi-surface instability. We identify wave vectors
that span nested regions of a band calculation and that match the neutron spin
crossover from incommensurate to commensurate on approach to the hidden order
phase.
