Server dissapearing off network

I have an interesting problem with a Windows 2003 Standard
server. I am using it as a network bridge for my ADSL
(Modem -> Server -> Router -> Wireless Clients), so I have
three NIC's installed. 2 3Com 3C905C's make up the network
bridge (I am using windows for the bridge) and another
Realtek card is simply plugged in to the router so the
server is visible to the network.

This all works perfectly for about 24 hours, the internet
is usable, server is usable, everything is fine. But after
about 24 hours of the server running, it suddenly
dissapears off the network. No machines can talk to it,
yet all connections are active with no errors. There is a
full set of link lights throughout the connections, but
the server is gone.

If I retsart the server, it works fine again for another
24 hours.

Specs:
- Intel P3 900 mhz CPU
- 256 mb RAM
- 20 gb HDD
- 512 mb pagefile
- 2x 3-Com 3C905C NICs
- 1x Realtek NIC

Any ideas?