Dear Julian ,
We started to get to use BMD cards only starting with MC 6.
Capture over 1394 (I tried SD ,PAL DV ) has the Audio buffer underrun issue after 10 -15 secs of capture when the BMD Card is installed and MC 6 is kind of automatically locked to it . As soon as the drivers are uninstalled and MC 6 does not see the card anymore ; Firewire capture is absolutley OK !!!
I am waiting for a reply from BMD ,and I have also made them aware of your case (this thread) to prove my point that this is really a known issue . Good to have someone to share the grief ; and hope it gets solved ASAP . Meanwhile also try out the 9.2B Drivers on the site as it say there are some MC 6 improvements . I am downloading it at the moment .
Regards
Krishna S