You would think that the abrupt and significant change in flight characteristics would mean a serious flaw that would be easy to find. I found that not to be the case and the frustration continued.

My only thoughts are from something I remember. It seems that after it hit that speed that caused problems I would have to re-trim the plane to fly at normal speeds. I noticed the elevons not straight after landing. I should have checked for airframe twist at that moment but can't remember for sure if I did. If yours is doing the same thing you might check it.

Something is really going out of wack when it happens because mine was like you bumped the elevons at least 50% up and right instantly when you hit a certain speed that was well above 100 mph. I wish I could help more.