Flutter most often comes from slop in the linkages. Way more often than from anything else. I've only run into a few servos that were sloppy around center, but that'd be the very first thing I'd fix and I'd fix it immediately. Are those servos new? If they're used, I'd swap them out in a heartbeat. Matter of fact, I'd quit reading this post right now and go do it. Right now.

There is something that should be mentioned here. Servo's usually draw the least current at center. And they also draw the least when they're under the least aerodynamic load. When your TX tells them to move that few millimeters down, they're no longer sitting at that sloppy center and probably not under the least aero load. Or maybe their gears are in an even sloppier orientation inside. Who knows...........