UFX arrived this week. Very exciting. Use in standalone mode no problem. Use with Totalmix control via USB or firewire to desktop PC no problem. Run up Nuendo 5.1 64-bit and attempt to playback mono track through it to stereo outs 1&2 via USB and Nuendo stutters (transport control visibly stutters) - can hear the audio out but also stutters as it follows the transport.
Tried all the USB ports (both 2.0 and 3.0) with the supplied USB cable and repeated the process with a different USB cable - same result.
Can see about 1 error per second on the error monitor - rate appears independent of buffer size
When plugging into some USB ports the Settings dialog sometimes shows multiple tabs (up to about 8) labelled as fireface(1), UC(2), Gain(2), LTC(2), etc. Windows 7 then reports that Settings has stopped working and its closed down. I can reactivate it again in Nuendo through Devices-Device Setup-ControlPanel but if I click on a tab it stops working again.
Then tried firewire interface. Selected max buffer size and just analog 1-8 - then it worked. Could reduce buffer size but not to minimum with it still working OK.
There are no other cards in the PC except for the graphics card - a Nvidia GeForce GT240.
PC is Intel i7 930 @ 3.8GHz, Motherboard ASUS P6X58DE, Drives IBM 160GB SSD + 1TB HDD, 12GB RAM
Symptoms suggest its a general bandwidth issue but PC spec should be good enough to handle it. I thought it might be related to interrupts which is why I tried all the ports . That still could be the issue I guess but any ideas on what to try are welcome!

General question on error monitor - does it have to show 0 errors continuously for the interface to work at all or could it still work OK with some errors?