i think my one nvidia based rig is having a similar issue. it's running 2.8c.

it will run fine for 5-50+hrs, then suddenly it crashes, gets the "unspecified launch failure" error, and doesnt auto-restart. i can see that the watchdog triggered, but then the pheonixminer.exe crashes in windows and it hangs until i manually close it. i usually just reboot the system and it's fine. i run all default parameters except only running the -coin eth command, everything else is defaults.

my AMD based system on 2.8c is rock steady. runs for 300+ hrs and never crashes.