If a dielectric slab is just made to enter within a parallel plate capacitor, then the subsequent motion of the dielectric slab is 

The electrostatic force that acts on the slab ( between free charges on the capacitor plates and the bound charge on the dielectric slab ) is not proportional to displacement. Rather it is proportional to the potential difference applied across the battery which makes the acceleration constant. Hence the oscillation is general periodic oscillation but not simple harmonic