Would this translate into sideways swaying? I have made the JFK - FRA trip three times in the -8, once in 2017 and twice this year. On the first and third trip I was stitting very far in the back, but only in this last trip I had the impression that the plane was wagging its tail all the time.
Unlikely - stab fuel is not needed or used unless you're approach the max aircraft range (which is much further than JFK-FRA), and then it's used early in the flight. The 747-8 doesn't try to do any CG management with the stab fuel.
Sounds more like a yaw damper problem.