Terminal Servers and Slow network speeds.

Hey all,
After going through a ton of posts and not finding a similar
problem, I figured I'd stop lurking and make my first plea for help

I've taken over operations for a medium-sized network, with all
Win2K3 servers. The company has around 400 thin-clients connecting to
three terminal servers. The terminal servers are using the MS Network
Load Balancing driver for load distribution and connectivity (I know,
not the best solution, but I need a stopgap until I can fix it up).

The three terminal servers only have one NIC each, and were set up
as a unicast cluster. As they added more users, network performance
began crawling, and yesterday they rebooted the servers and lost
cluster connectivity. I disabled NLB on the NICs, and the servers could
ping and be pinged. Eventually I read up a bit, and reconfigured the
cluster as multicast, and everything came back up, and transfer speeds
were great (although this may be because no one else was connected).
Unfortunately, a few of their remote sites use Cisco routers that don't
support multicasting, so I set it back as unicast, and everyone could
connect, although transfer speeds were slow.

Now, after hours, there's no one connected to the terminal servers,
I've disabled the NICs on those servers completely, and am just
troubleshooting, but transfer speeds are variably slow; that is,
depending on which direction I'm sending files, and which
machines/servers are involved, the transfer is either full-speed, or
crawling.

Any ideas or suggestions? I've looked through a ton of
documentation, but nothing seems to really apply. I appreciate your
time, and look forward to a response.