Firewire is the reason.
There's no way that you're going to achieve sync with the computer monitors and an outboard HD monitor being fed via firewire. It's always gonna be around 5-8 frames out. That's basically enough to make editing impossible. (You're trying to mark in on the fly and you're always off)
The only solution that I know of is a "card" based in/out like the Kona 3 or Blackmagic. These use the faster internal bus, so there is no delay.
Mark