[expert] Network unreachable

[Nicolas Le Gaillart]

I've got a very strange problem with an 3com 905B Ethernet card
(Boomrang) : The network connections starts to work fine, but stop after
a while. (a ping or a route tells me that the "Network is unreachable"
!)
Then, in order to make the network connection work again, I have to
reboot the machine, which launch again the network services... 
The network connection is completly stoped : a ping from another machine
don't reach my linux box !
Please help me ! Pleeeaase ! I'm getting mad...