I just started using my new Mac Pro and I too have that 7 frame out-of-sync thing happening in FCP. I capture on the fly and all looks good. I edit within FCP the next day and all looks good. I come back to it the next day, and the audio is off by seven frames. And it is intermitent and thus far unpredictable.
I re-sync the project, and the next day it is off again, whether or not I left the computer on overnight (GASP!) or not. Occasionaly it will happen if I close FCP for a while then return. What the…?!