I'm having the same problem with a Lenovo W520 (i7) laptop, running Win 7/64 with a Ricoh Firewire chip - sometimes works, but usually loses sync as soon as I start Nuendo. The Fireface 800 is not then recognized unless I reboot the computer.
My workaround is to use an expresscard Firewire adapter. The FF800 work fine. It is recognized the instant I plug it in, whether Nuendo is running or not. If I use the built-in Firewire, the FF800 will drop sync within a few seconds.