I just heard from Mamiya rep in Taiwan that the firewire problem with ZD is not in the camera or in the cable - it is in the way the cable is positioned: if cable is circled, the camera stops working in firewire mode, once cable is straightened, everything should work fine! I will try it with cable which came with ZD.
On the other hand, I have used a firewire cable, purchased from cablestogo, when shooting tethered with Fuji S3. That cable has no magnets at the ends. Cable was literally abused, tangled, detangled, wrapped around the camera mounting gear, etc. and it still lives and never gave me any problems.
I suppose Mamiya's cable has bad shielding, so it interferes with itself. Circles of cable create inductive coils which behave like electromagnets and are susseptible to intereference.