Do you think that the air across the pipe could be acting like part of the right fin and add extra pull at higher speeds. I have a macs muffler available that I could put on it to see if there is a change but the down side is that I may not reach the speeds with the muffler that causes the problem.
The DD has never had anything close to a hard landing or abuse. I can lay it flat and check every gap and even thickness all over with calipers and it is good. Maybe the covering is tighter on one side than the other. I'll work on one part at a time to resolve.