Flying in an a319, got up to FL330 and have the AP set up as I always have, but when the aircraft reaches the cruise alt after a slow climb, it is stable for a short period, then starts to ‘bob’. The weather isn’t anything drastic at all, but the aircraft will slowly begin to rise, then descend, then rise, and so on and so forth. After a while becomes rather profound pitching and nose diving. Ends up to around (-)900ft/m.
Anything obvious I’m missing?