Problems with 3c920, vortex and a 10baseT only router

I have just installed RedHat 8.0 in a Dell Latitude 840 with 3c920 on board.
I have a direct ethernet connection to a DSL router (10baseT only) that
works perfectly with the same machine with Windows XP o with other machine
with RedHat 8.0 (and a 3Com Fast Etherlink XL 10/100Mb TX Ethernet card).
Also, I can connect one machine to the other using 100baseTx.

The problem is that I cannot establish a connection from the portable
computer to the router. I have tried to force the 3c920 card to 10baseT
(because I can connect perfectly to the other computer using 100baseTx and I
figured that the problem could be the negotiation), but the only thing I can
get is this:

[root@Quevedo mamarin]# mii-diag -A 10baseT-HD eth0
 Setting the media capability advertisement register of PHY #24 to 0x0021.
Basic registers of MII PHY #24:  1000 782d 0041 6800 0421 0021 0004 2001.
 Basic mode control register 0x1000: Auto-negotiation enabled.
 You have link beat, and everything is working OK.
 Your link partner is generating 10baseT link beat  (no autonegotiation).
   End of basic transceiver information.

[root@Quevedo mamarin]# ping 10.0.0.1
PING 10.0.0.1 (10.0.0.1) from 10.0.0.4 : 56(84) bytes of data.
64 bytes from 10.0.0.1: icmp_seq=19 ttl=64 time=0.642 ms
64 bytes from 10.0.0.1: icmp_seq=21 ttl=64 time=0.631 ms
64 bytes from 10.0.0.1: icmp_seq=22 ttl=64 time=0.633 ms
64 bytes from 10.0.0.1: icmp_seq=23 ttl=64 time=0.641 ms
64 bytes from 10.0.0.1: icmp_seq=27 ttl=64 time=0.642 ms
64 bytes from 10.0.0.1: icmp_seq=32 ttl=64 time=0.655 ms
64 bytes from 10.0.0.1: icmp_seq=33 ttl=64 time=0.649 ms
64 bytes from 10.0.0.1: icmp_seq=36 ttl=64 time=0.646 ms

--- 10.0.0.1 ping statistics ---
36 packets transmitted, 8 received, 77% loss, time 34999ms
rtt min/avg/max/mdev = 0.631/0.642/0.655/0.023 ms


If I try to force with mii-diag -F I get the same results. Any ideas?