Are you flying between OPT (OPTimum altitude) and MAX (MAXimum altitude), according to the VNAV CRZ page (page 2/3)? Or above MAX? If you are flying above the MAX altitude, then you're flying too high for the weight of the aircraft, and a dutch roll is bound to happen.

I always try to fly at an altitude close to the OPT altitude on long haul flights, and generally quite a bit lower on short haul flights. If I'm doing a long haul flight, I often do 2,000 foot step climbs, by climbing to the new OPT altitude once it reaches 2,000 feet above what it was before. i.e. flying at FL340, when that's the OPT altitude at the begining of the flight, then climbing to FL360 when it becomes the OPT altitude, and so on up to the maximum altitude the aircraft can fly at.

Note. Step climbs are normally at 4,000 foot intervals.