I've noticed over the last several weeks that when I load a project it will play either twice as fast or twice as slow as when I put the project away. This could be only a matter of hours between putting the project away and re - loading the project. This happens with project files or loading stereo files into mastering projects. 
This is a new anomaly I would place around the time of the July updates. I have found that by quitting Sonar and relaunching it can fix the issue.  However, today I am loading a stereo file to work through a mastering chain and no amount of launching Sonar is addressing the issue.
 
Has anyone else run into this tempo inconsistency problem?

Check your Playback and Record Timing Master settings under Preferences > Audio  > Driver Settings, and make sure they are set to channels of your interface. You might also want to uncheck 'Share Drivers' under Playback and Recording.
 
Related to this, a lot of people seem to feel that they have to disable onboard sound to avoid interfering with their dedicated audio interface, but I recommend the opposite: Leave it enabled, and set it as the Default Device for Playback and Recording in the O/S so that Windows and generic multimedia apps that aren't specifically configured to use your interface's ASIO driver keep their mitts off of it.

brundlefly
Related to this, a lot of people seem to feel that they have to disable onboard sound to avoid interfering with their dedicated audio interface, but I recommend the opposite: Leave it enabled, and set it as the Default Device for Playback and Recording in the O/S so that Windows and generic multimedia apps that aren't specifically configured to use your interface's ASIO driver keep their mitts off of it.

I do this too. It prevents Windows system sounds from playing thru the audio interface.
I have the onboard sound output available in my patchbay if I need it.