Dennis, I had the same problem once & it confounded me for WEEKS. The fix?
I accidentally unplugged my firewire hub from the power, and when it was
plugged back in, the problem was gone. It's like it needed to be "flushed out" or
something. That's not the only thing that can cause it, but if you've
exhausted everything else, just give it a shot. Ya never know.
Kerry