I've seen this a few times, and here is what I usually try:
1. Shut down the back & C1, disconnect the firewire cable, and power off the back. Reboot the hardware and software, and then reconnect the cable.
2. Try a different cable.
3. Replace the battery in the back/camera. (I know, that doesn't make sense, but it's on my checklist)
4. Try a different firewire port. (if possible)
5. Remove all other firewire devices from the firewire bus so that the camera is the only thing on the chain.