We have one BCF in our company which displays this behaviour as well. After a lot of troubleshooting the engineers came to the conclusion that one of the wires of the upper yaw damper was corroded. For roughly two years did indeed fixed the problem but it is coming back now at times. Today at 300 kts and roughly FL230 - FL 250 the rudder output on the CMC display page did not show any unusual fluctuations.
Just getting more and more mysterious about this Boeing papers around this phenomena if they exist