This happens frequently.  I have been working on a project.  Then, either suddenly or when I return from a break, hitting PLAY will only play the first few measures of the song and then Sonar will stop.  If NOW is past the stopped point, the song won't play at all.
 
It is as if somehow, some kind of STOP marker got placed in the song.
 
Restarting Sonar and/or the PC will usually fix it.
 
Thoughts?

This is a stretch, but sounds to me like virus scanner.
 
If you make a long recording, can you replicate the problem? like record 16 tracks for an hour. hit stop and then try to start playback again?
 
Sometimes, when a computer has been idle, background processes start to run. You could probably detect the process by checking task manager.
 
Edit: In Power Settings > Advanced > USB have you disabled Selective Suspend?

I can't find that thing about USBs.  In Control Panel, I have Power Management, but USB is not addressed there.

Konrad,
 
I'm using Win 8 but I think it's pretty close...
 
I hit start and type Power Options and launch that...
Then it comes up and it has High Performance checked...
I click change plan settings
a new window opens with Change Advanced Power Settings link
This opens a new Power Options window with advanced settings that includes the USB Settings.

+1 - The above instructions should get you to where you can turn off the selective disable functionality for the high performance power plan options.
 
If you still cannot get to that to change the option, there is another way to do it.
 
In Windows 7, hit the Windows button, then right-click on Computer, and when the drop-down menu opens click on Manage.  This will open the Computer Management window.
 
In Computer Management, look on the left side, and click on Device Manager.
 
Once Device Manager opens, scroll down to and click on the little arrow to the left of 'Universal Serial Bus Controllers' - this will expand that node and show all of the USB devices.
 
OK - now one at a time, double-click on each HUB displayed.  (If it is not clear from the name of any of them whether or not it is a 'hub', double-clicking to open them doesn't do anything except open up the Properties window for the ones you open with the double-click).  When the Properties window opens for each USB hub device you open, you want to look at the top for a button/tab called 'Power Management'.  (If you do not see the Power Management tab/button for a given device you have opened, it simply means that this device is not a hub, and you can close it and move on to the next one).
 
OK - so, for each Hub you open and see the Power Management tab for, click on the Power Management tab, and it opens a sub-window that includes a a check-box option that says: "Allow the computer to turn off this device to save power".  By default, Windows has these checked, and by removing the check from the box, you ARE turning off the Selective Suspension of USB devices.  This is just another way of getting to the maintenance of that same Windows functionality.  So, after removing each check box from each USB hub, click 'OK' to complete the alteration of this functionality, and then you can close your way back to the list of USB devices and move on to the next one.
 
Hope that helps, 
 
Bob Bone