Thanks for testing this @howil and I appreciate the detailed reports 
It’s difficult to conclusively identify high speed signaling problems without specialized hardware. If we land on any firmware build that solves the issue, it would of course confirm that we could solve the problem with firmware. But until we’ve exhausted all possible avenues in firmware, we probably won’t be able to say for certain which it is.
If you do happen to have a friend with a 2GHz+ scope (very expensive), that’d be very helpful, but of course I would not ask you to go to those lengths unless you really want to 
If you’d like to discuss options for your device, please contact support@puri.sm, and you can mention that we have already troubleshooted the issue to an extent on the forum (please link the thread, it’s not always clear how to associate email addresses with usernames).
I’m still investigating more possibilities for USB tuning improvements, but progress is pretty slow both due to the nature of the issue and other issues I have been working at the same time. I greatly appreciate your patience and feedback!