The long-term rotational evolution of the old, isolated 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 large internal temperature to pinning energy ratio towards the base of the crust implied by the recent high surface temperature measurement leads to linear creep interaction to operate in the densest parts of the inner crust where vortex lines assume a parabolic shape due to pinning to nuclear clusters. The resulting low frequency oscillations of vortex lines combined with the time variable superfluid-external pulsar braking torque coupling give rise to 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.