My /etc/modules.conf looks like this:
...
alias eth0 3c59x
alias eth1 eepro100
options eepro100 full_duplex=0
Login as root and use vi or another editor available to add an options line.
My eth0 is the internal interface. it runs at 100MBs, Full Duplex on autodetection.
My eth1 is the external interface connected to the dsl router. The added options line specified NOT to use full duplex. It works in this driver's case.
I am not sure which NIC you use or which driver you use. However, you can try this for starters. Look on the vendor's web site for any information for switches available for your linux driver. You may be able to find something.
I looked on the deja.com web and did a search in the comp.* newsgroup with the keywords eepro100 and full duplex in order to find this suggestion. It worked for me and I have not had a drop from my dsl connection since!
Hope it is clear enough.
Serge.