Audio Dropout/Freezing During Loop Recording

I'm experiencing freezing and complete audio dropout when I'm loop recording.  It records fine up until the loop point but then it freezes and I experience a complete loss of audio unless I shut down and restart Sonar.  It appears to be playing but there's no sound after this point.
 
I can playback looped sections, it's just when I try to record in this mode.
 
I'm running Producer Expanded under Win 7 via an RME Fireface 800. 
 
I've tried increasing the buffers in the FF800 to max but I still get the same issue.
 
Does anybody else have this problem or can anyone suggest a fix?