The next time you get the black screen problem, try unplugging and replugging the firewire cable.

If I understand things correctly, this will essentially "reboot" the firewire and video capture drivers without the need to reboot the entire computer.

Trying VirtualDub was a good diagnostic step. It narrows the problem down to the PC's drivers, or something external to the computer such as the video camera, its firmware, or the cabling.