I would try going the PCI or PCIe card route (PCIe if you can). Just make sure you get a card with a Texas Intruments chip. For example, this one from Scan. It's only £20-odd quid, and Scan products are usually reliable.,
Don't run anything else through the same Firewire bus as the Fireface. And definitely try a new FireWire cable if you haven't already -- the connection between the PC and the Fireface going down could indicate a problem with the cable.