That's the point. Substitution is a valid eliminator.
If a connection is mostly reliable for years, then suddenly fails repeatedly. And the user then simply switches to another network - eliminating the drops. I'd say that pretty much excludes problems on the user's end? Everything that could be done here to mitigate/rule out local problems happened.
"Substitution test" is *still* running without failure. Supplier might technically be a small step down - but it works.
Proxy platform is a Thinkpad T41 Win 7 w RollbackRx running from Guest account which has been used for years with various VPN providers. Very dull - just works. Until now.
Switching networks fixed things instantly.