A few years back I rode along on several spins flights in a Katana, and clearly noted some very slow pitch oscillations during an otherwise stable upright spin. I didn't think much about it at the time, but I'm sure that the gentle amplitude and long period could not have resulted from something as major as unstalling - restalling - etc.
My personal theory would be some small change in the flow field around the wing or stabilizer, changing the pitching moment of the airfoil, but leaving the wing well and truly stalled.