Definitely not happening here on Win 10 with my little Saffire 6 USB.
 
I have left this system running unattended for hours, come back, press play and it plays instantly.

Not happening here with the TASCAM US-20x20 USB 3.0 interface.

Noel Borthwick [Cakewalk]
I've been seeing this occasionally with the RME UFX. It seems like after the system has suspended and resumed the driver won't emit audio. I close and restart SONAR and it plays again. It actually happened to me today. 
You could try contacting RME about it. I don't think SONAR is in the equation since our meters are showing activity and audio is streaming as normal.

Noel, this does not happen like a flick of a switch. This seems to get the audio buffers corrupted over a minute or so. I was playing the file, got some distortion and the distortion got worse until there was no audio.
 
Any other suggestions?

I tried leaving an audio clip sitting in Sound Forge and I did not experience the distortion or missing audio. I let it set for 5 hours without issue. 

How can I say this is not a SONAR/RME issue? I don't think I can at this point. Running the test with X3 now.

Before you mentioned that after 5 min you had the problem. This is not something I have heard about or seen. You have Win 10 is it Windows Pro 64 bit? You may want to say what build you're using.  Also what driver are you using ASIO or WDM? try using a different one. Play with your buffers settings too. 
 
This has to be highly frustrating for you and wish I had an easy answer.  

Thx John. It is a 64-bit standard version and ASIO drivers. WDM doesn't work for my UFX. Just let X3 sit for a couple hours and not exhibiting the issue in X3. This looks like a SONAR Platinum issue with RME and Win10 64bit. Whatever the default update version of Win10 is. 

Conley, I'm not home and have no access to a win10 computer where I am, but I had a problem such as yours, and I changed a setting in the advanced power area that controlled power to USB ports ? It was timing out after 120secs or something like that. I can't remember exactly what the path was, or how you get to it without a machine in front of me, but it fixed my issue, which was after a substantial win10 update about 6weeks ago?? Maybe someone else can tell you exactly what this setting is. I won't be home for a few days. Cheers - Dave.

thx for the reply Dave. I looked at the USB Hub settings and the box to allow the computer to turn the device off was unchecked. The UFX has it's own hub and is high speed.

I am going back to windows 8 because SONAR doesn't handle this audio engine thing very well in Win10 on my machine. RME says nothing is wrong because the driver works in other hosts and other versions of SONAR. It is a platinum specific issue.

I've had it happen to me a few times lately and I'm on Win 8.1 and the latest version of Platinum.