I have similar issues and sometimes my pc actually reboots on its own. The headphone output is really 'dirty' one day and works the next. I have Win7 x64, Sonar X1,2,3 (all have the same issues) All drivers are up to date with green checks in Line Monkey.
I have rolled back the UX 2 drivers to prior versions which did not make a difference. I have reinstall drivers (currently v4.2.7.1), the firmware (currently 1.01), and tried multiple USB cables and ports and still have the same unpredictable performance from the UX2.
I have also reinstalled Windows 7, and recently upgrade my DAW to the latest version to see is that was the issue. I also removed Google Chrome because it cause my pc to reboot on it's own recently.
...at a loss!
