Clock sync issue with XI and Line 6 UX2

Hello all,
I've been in and out of these forums for years now, and have found them to be an excellent resource with very little nonsense (hard to come by on the web these days). Thanks.
 
I'm a part-time amateur musician, and I've been using Cakewalk since the early days (DOS).
 
I'm now on Sonar XI -64 on a new Intel I7, running Windows 7.
 
My sound card is a Line 6 UX2. I fully appreciate that this isn't the best card for audio recording, but I had no problems on my previous setup (XP with Sonar 4).
 
My problem is related to Audio clock.
I can play back both Audio and MIDI.
But I can't record MIDI unless I do one of the following:
Change Clock Source to internal. I can then record a MIDI track. However, as soon as I play it back, I can no longer record a second track, as the clock source has been automatically switched back to Audio;
or Record an Audio track at the same time as MIDI.  
The symptom is that I can arm a MIDI track, press record, and then SONAR appears to be in Record mode, but the clock doesn't start, as if it's waiting for a sync from somewhere.
 
I'm using the Line 6 Asio drivers. I've tried playing with buffers, switching to WDM, and disabling the internal sound card. Nothing helps.
 
Weird. I've checked all the usual suspects (http://www.cakewalk.com/Support/hwsetup.aspx?dm=asio&man=Line%206), and can't find a solution.
 
To summarize - I can record Audio fine, but MIDI doesn't record unless I either (temporarily) switch to Internal Clock, or record Audio at the same time.
 
My MIDI device is a MIDISport 2x2.
 
Thanks for any suggestions you may have,
Andrew