FW400 has a theoretical throughput of ~ 40 MB/s, which will in fact give you ~ 30 MB/s (equals ~ 170 streams of 44,1 16bit stereo samples) adding about 10% CPU load ... so we shouldn't see any bottleneck here
more probable is a load coming from the GUI, especially if you have a large screen ... we noticed already several times (especially on PPC macs) that GUI ressources are forcing a lot from the CPU .... try to minimize interfaces to see if we are on the right trace ...
network and firewire shouldn't collide (unless you are using TCP/IP over firewire) and you don't put more load eg. on a 100 Mbit network as it can handle
you should also avoid to have a FW audio device and disk on the same controller .... this will most probably produce dropouts
christian
and remember: only a CRAY can run an endless loop in just three seconds.