Hatstand
probably a dumb suggestion as you seem to be extremely thorough, but have you run LatencyMon?
Haven't used LatencyMon in ages but I'll give it a shot again.
pwalpwal
maybe the latest driver will fix it (3118 is newer than the one you posted)? drivers were updated recently https://www.rme-audio.de/en/downloads/driver/fireface.php
Double checked it. Windows Device Manager reports 3.1.18 while the Fireface.exe (systemdir) still shows 3.1.16, hence the version mentioned above. I check their drive page weekly so I knew I had the latest downloaded. That indeed may be an issue but so far Sonar is running completely stable in Compatibility mode for Win8.
I also went through every driver model available, switching from ASIO to WASAPI, to WDM/KS and never saw the stability return, the RTL just went through the roof (which would be fine if I were mixing).