Well, I figured it out. Or, rather, my ACSR did. James Beattie at CTG. A really good guy who knows his stuff.
I have my Avid hooked up to my home network via ethernet on a router firewalled from the internet. Apparently, even though the Mojo exists on its own firewire bus (the internal one), this bus still competes with the Ethernet card for bandwidth/resources. Unplug the ethernet cable (or disable the network using My Computer) and, voila, no more DMPort Errors. Never happens again.
James went on to tell me that if I swap the hardware between firewire cards (Digi 002 and firewire drives on the internal card and Mojo on the installed card) all will be well, too. Even though Avid recommends Mojo on the internal, as long as they are seperate, no problems or conflicts.
Hope this helps anyone else who might be fighting this bug.
BTW, Avid never did speak up about this problem, even though I posted here, on the Avid-L, Avid Commuinty Forums, et al.
Cre8tive