Are coming simply idling. It seems that just leaving the DAW sitting with a project loaded for an undetermined amount of time takes me to 'the audio engine has stopped unexpectedly. It's done that at least 4 times in the last 2 days. Running 2016.5. Only new plug or software is Soundtoys Echoboy installed and used last week a number of times without this effect on the system. Thanks, Shep
 
Edit - I am running all current software versions. It did it again as I looked to ensure 'up-to-dateness'.

Are you on line... Windows updates and all sorts of background issues can cause this. 

Hi Shep. I would add to what Johnny said that updates can be an issue not only whilst they are happening, but also by what they update. If taking the computer offline doesn't help you try rolling back to a system restore point prior to the last windows update or when the problem began.

There is a default Windows Advanced Power Management option called USB Selective Suspend, and it is turned on by default.  It allows Windows to turn off individual USB-connected devices to conserve power, and this can be quite a pain in the booty for things like a USB-connected midi controller or audio interface.
 
You can check and change this, by launching the Windows Control Panel, then click on Hardware and Sound, then click on Power Options.  Once there, make sure the Power Plan is set to High Performance, and then click on Change Plan Settings, and then Change Advanced Power Settings.
 
Once there, scroll down to and expand USB Settings and expand USB Selective Suspend.  Make sure that USB Selective Suspend Setting is set to Disabled, then click Apply, and OK your way back out - if it offers you the chance to save your applied settings by all means do so.
 
The above will help Windows from disconnecting USB devices that Sonar may well rely on, as things don't wake up well in Sonar once they have gone to sleep or hibernated.
 
Bob Bone