Hi. I am running a RME Ucx, latest drivers (USB), latest firmware, latest SONAR and Win10 X64 (fully up to date). I do not have any problems with the audio engine cutting out when leaving SONAR for hours, or even overnight. 
All the best.

thx for the replies everyone. 
 
Power settings are set for performance and the ports are not set to be be turned off. The UFX is plugged into a USB2 port. I would imagine that the latest updates would have been applied when Win10 did the upgrade. I could check that out and see. I don't want to do the update and get stuck with win10 if this problem persists. 

Now I am beginning to wonder if this project is helping cause the issue. All tracks are muted and I am only solo'ing the master mix down track. So only one track is active. which would still point to SONAR.
 
I didn't see anything pointing to SONAR in the error reports.
 
I will try again and see if I can get any closer. I will take it offline and see if that matters or not. This is 100% reproducible for me in Win10 and platinum though. Works fine in X3.
 
 

I'm running the UFX on Win10 using the drivers that were available at Christmas when I rebuilt my laptop and I have not seen the issue.  
 
As Jim said I would check the power management settings.