  The long-term rotational evolution of the old, isolated pulsar, PSR B0950+08
is intriguing in that its spin-down rate displays sinusoidal-like oscillations
due to alternating variations, both in magnitude and sign, of the second time
derivative of the pulse frequency. We show that the large internal temperature
to pinning energy ratio towards the base of the crust implied by the recent
high surface temperature measurement of PSR B0950+08 leads to linear creep
interaction between vortex lines and pinning sites to operate in this pulsar.
Vortex lines assume a parabolic shape due to pinning to nuclear clusters and
finite tension of vortices acts as a restoring force that tends to bring a
vortex back to its straight shape. The resulting low frequency oscillations of
vortex lines combined with the time variable coupling between the internal
superfluid components and the external pulsar braking torque give rise to an
oscillatory spin-down rate. We apply this model to PSR B0950+08 observations
for several external torque models. Our model has potential to constrain the
radial extension of the closed magnetic field region in the outer core of
neutron stars from the oscillation period of the spin-down rate.
