I have two FA-66, because my first one worked absolutely 100% dropout-free:
- PowerStudio660 (= green/black coloured FA-66 known as an SPS-66 and branded as Cakewalk hardware, bundled with full version of Sonar 6)
- WinXP 32 bit pro
I bought my second FA-66 (darkish red coloured Edirol branded) cheaply, looking to double my microphone inputs. Daisy-chained/cascaded together, they also worked 'in unison' absolutely 100% free of dropouts and crashes.
I then upgraded to X1 and Win7 64bit, because my former PC, assembled in 2003, couldn't handle many MIDI tracks and effects (CPU load too high, limited RAM). Now I keep having problems with dodgy things in X1 or Win7 - often, nobody seems to know why(?), but the FA-66 itself hasn't let me down, yet. I have, however, not been brave enough to daisy chain them (I was over-optimistic when writing my signature), for fear of getting even more problems. I think I remember reading in the notes on the Win7 64 bit drivers for the FA-66 and SPS-66 that their support is 'limited' for Win7.
So, based on my experience with two almost identical units, I too believe that your FA-66 may not be the culprit. Rather, their interplay with either Win7, 64 bit, or X1 and the FA-66 drivers is the problem - but that of course is even more irritating, because you can't solve any of that easily by yourself ...