Horrible ethernet performance under mild load - SB75G2

Hello everyone;

I have a Shuttle SB75G2 XPC with the onboard Broadcom SCM5788 gigabit
ethernet port. I have a Seagate Raptor 74G SATA drive (OS), a Wester
Digital 200G "Drivezilla" PATA drive (storage), and I'm running
Windows XP SP1.

My PC is connected to a Linksys 10/100 switch (not hub in case it
matters). I also have a mid-end Windows 2K Pro machine with an Intel
Pro 10/100 nic connected to the Linksys. Finally I have a Redhat 9
server with 2 Intel Pro 10/100 nics, one connected at 10Mb to the
cablemodem, one connected to the Linksys. I'm using iptables+snat to
masq an rfc1918 private network (the two windows machines) through the
cablemodem via the Redhat server.

When I'm using Newsbin Pro (4.x - registered) to suck down tons of
binaries on usenet at 3-3.5 Mbps over my Cox cablemodem, my
windows-to-windows file transfers slow down to about 1.5 Mbps.

When I use the option menu in Newsbin Pro to set the bandwidth
limitation from NONE to 500 Mbs my windows-to-windows file transfers
speed up to about 48 Mbps.

When I allow Newsbin Pro to download at around 1 Mbps my
windows-to-windows transfers slow to about 8 Mbps.

When Newsbin Pro isn't downloading, I get a windows-to-windows
transfer of about 75 Mbps.

My suspicion is the fact that the Broadcom gigabit chip is sharing
IRQ15 with the IDE controller(s) the Shuttle can't handle the load
between disk access and ethernet throughput.

Does anyone have any ideas on how to make the Broadcom chip perform
full speed under any load?

Thanks!