Well, this could be a power station switching issue. Call up your power company and ask them if they are doing anything at this time. They often do not monitor power quality closely enough to tell when they have bad switching issues.
It sounds like your DAC is switching to the wrong sample rate or a similar problem. This can happen if the power has been interrupted or it gets a surge from somewhere.
Another source of a problem is a scheduled job on your PC that causes this, such as your anti-virus refreshing. Check the logs and see when it last updated.