I've installed a playback system at a facility, using  two Fireface800s.

The playback computer is running Nuendo(ver.5.51, build346, 64bit).

The computer's spec is : Windows7, Pro service pack1.
                                  Intel i7  2600M, 3.4GHz,   RAM 8G.

RME driver version 3.047, Hardware version 2.77.

On Nuendo, they're using 16 tracks, individual track goes out of analog output of two FF800. 

At times, serious noise arises. Sounds like buffing noise or something like.  They've tried to change buffer setting but the problem         

still exists.

If they restart the Nuendo program, the problem doesn't happen.  Between the shows, they always have to restart the program to

avoid the problem.

And Nuendo is run by receiving time code from video playback computer. Nuendo loses the timecode at times, it happens very

randomly. 

What could be the possible reason?   I'm not even sure if this problem is more related to Nuendo program or FF800. 

Help me, plz...