Took a while to solve this so I thought I'd post for anyone else hunting.
Persistent clicking with UX2 on Win8 x64 (AMD A10 + discrete gfx, SSD & 16G). The system runs plenty fast enough, so I was surprised to be getting drop-outs & clicks in all playback apps, especially on basic activity like switching app focus. Tried adding ASIO driver to Foobar2000 & adjusting buffers, latency and bitrates in the Line6 control panel and everything with no success.
Culprit in the end: the UX2 was plugged into a USB 3.0 port. Plugging in to a USB 2.0 port fixed the problem.
Unsure if a particular controller was the problem (Gigabyte F2A85X-UP4 has both native AND A85X chipset and Etron EJ168 controllers). Bought a board with extra USB3 ports thinking I was doing myself a favour -- only 2 USB2 ports on the back panel -- jeez. LOL
Anyway, fixed -- hope this maybe helps someone.