Sometimes switching to WDM mode and running the audio utility that Sonar does on start up of MME and WDM mode will often resolve cranky driver behavior. Now just switch back to ASIO. 
I do this when ever I install Sonar and run for the first time. I'll run in WDM mode just to have that audio analyzer do it's thing. Then swicth to ASIO mode. 
 
You might want to check your USB cable is not faulty.. wiggle it.
Or that you are not using a USB port that falls asleep. ( power saving feature) 
And if the problem becomes that Sonar keeps switching to your on board sound card then I would try disabling the on board audio in device manager. Also read Craig Andertons tips and look for the part about Video cards and audio.