I'd be very surprised if you were dutch rolling in an A320.
I've had similar conditions to what you describe when sitting in someone's wake at the same level. Occasionally when you intersect the same track at an acute angle, you can end up in a situation like this for a couple of minutes until you finally escape their wake as you head off on different tracks.
The airbus autopilot system has a tendency to wallow about oh so slightly on autopilot, unlike the Boeing which is a bit more 'pinned' in making corrections to deviations in altitude.
It might not be this, but it's a possibility!