two NICs and speed issues

[sam at illawarra.org]

Hi,

i'm running Fedora Core 2 on my server - which, for the most past works
quite well.. kudos to the dev team. However, i have a minor problem i
cannot seem to fix and is thus:

i have two network cards ( sis900 and usb200m ) and the first is connected
to my dsl300+ (dlink) modem at 10mbit and the other is connected to a
10/100 switch that feeds the local lan. if i unplug the dsl300+ from the
rj45 my local lan works quite fast 8MBytes ftp transfers etc to the local
net via the usb200m. however if i plug the modem in i only get 120kbyte
transfer speeds and such. i have switched the modem to the other nic and
the same thing happens. for some odd reason i can only get good speeds to
/ from the server to the local net if i only make the local net nic 10mbit
with mii-tool OR unplug the modem and thus get 100mbit speeds..

help!

regards,
Sam.