I had this happen on my last flight in the 707.  I was climbing through FL280 for FL300, and was adjusting the A/P pitch controls downwards, in preparation for the top of my climb.  The plane suddenly started porpoising through the air, with the elevators deflecting full up and down, rapidly.  I immediately disconnected the elevators from the A/P, and stabilized the aircraft manually.  When I attempted to reconnect the A/P for altitude hold, the plane suddenly and violently pitched upwards, (we're talking 6000fpm of upward pitch) and stalled.