Linux box causing MSN hiccups?

Hi all,

I have a router/modem (Dynamode) currently serving 3 computers on XP and
one, my file/web server, on FC4. The server remains on at all times but with
no-one logged in, just running the services, not running any GAIM or such
like. MSN on the windows machines is showing odd symptoms of losing messages
completely for periods of time and occasionally diconnecting. When i had the
linux box being the gateway, with a USB modem, i had no issues but in this
configuration i do. I have unplugged the server and it seems to have cured
my problems. Any ideas why this would be? How could the server be upsetting
the MSN on the windows machines?

Cheers,
James