  The vortex response to various ac-drives has been studied in high quality
$YBa_2Cu_4O_8$ single crystals.
  Well within the vortex solid phase a re-entrant resistive state has been
observed that is characterized by long relaxation times. Pulse current response
and IV characteristics reveal that in this state the driven vortex system
relaxes in a way to increase t he effective pinning force. A phenomenological
model accounts for the observed features, in particular the clock-wise
hysteresis of the IV-curves and the increase of the apparent critical current
with applied current amplitude.
