HF suspects
Through a process of elimination I now suspect that it might be the stand on which the lower end of the antenna terminates. I am guessing that this is shorting internally and causing the issue.
If I read the picture correctly that post could be just about the right
length to cause issues if it is alternalty shorted/open with vibration.
The clean flights can be explained by tensions changing a bit after the antenna was swapped.
When you do replace the post keep a sharp eye out for where something might be making intermitant contact, does not matter if it is supposeded to be grounded and coming loose or meant to be isolated getting grounded, either one could do it.
Thanks for keeping us posted, this is indeed one of the more fascinating issues I have followed in a long time.
BTW: Suspect the issue was just waiting for you to claim victory before re-appearing
