It's possible to get out of synch with the autothrust by over controlling. For example, the aircraft sinks slightly and you over correct by applying too much back pressure.The A/T kicks in and you start going above the profile, you apply forward sidestick and the power comes off. As you get back onto the profile you apply back pressure and the A/T comes in again causing you to climb etc etc.
Small corrections and anticipating slightly will usually cure this, but as stated earlier, if the automatics aren't doing a proper job then disconnect them and fly manually.