I have a server running Debian stable, which functions as a router/firewall and mailserver. It has two NICs, both realtek 8139. eth0 is connected to my LAN and has ip 192.168.0.1. eth1 is connected to my ADSL ethernet modem and has ip 145.94.196.59 which it gets through DHCP and is dynamic.
I used to have the same setup on another server where it functioned perfectly. That server had basically the same hardware but used a 3com ISA NIC for eth1.
Since I'm using this new server, which is a little bit faster, the internet connection sometimes fails on me. First I thought it might have something to do with the DHCP, because I sometimes noticed I had no valid leases. This is not the case however, since the lack of valid leases is due to the fact that the internet connection fails.
I suspect it has something to do with the whole network, because when I have a SSH connection in my internal network (thus to eth0) opened, this connection failes too. The eth0 revives though, since I can open a new SSH connection immediately after.
I have found that running /etc/init.d/networking restart brings the network back up. Also when I run and close tcpdump the connection is back to life.

Does anyone have any idea what might be going on here? It totally beats me, and if I don't cure this very soon my housemates might beat me too for their internet not working properly
Any help would be greatly appreciated!

this may seem silly (but you'd be surprised how many times it happens), but check to make sure that no two machines have the same IP address within your LAN. And then make sure your two ethernet cards are not occupying the same IRQ lines, (especially the same IRQ PINS) - this caused me some trouble in the past.

The problem of two machines having the same IP does not occur, I checked and double-checked. I don't really understand about cards using the same IRQ lines and IRQ PINS, how do I check if that's the case?
I tried switching the cables and reconfiguring so that eth1 became eth0 and the other way around. Now the internal network fails, so the same NIC! I thought it might have something to do with that nic, so I replaced it with another Realtek card and put it in a different PCI slot. Still no luck though.
Any ideas? Thanx

To see IRQ lines/pins: scanpci -v as root (note INT_LINE, INT_PIN).

When you are able to access the internet from within your LAN, ping some site on the outside (like google.com), keep the ping going and note what happens when the service fails. I suspect that it may not be a total failure (since it occurs intermittently) , but that a massive packet loss occurs. It could be poor firewall settings, or you may need to tweak some kernel settings.

Thanks for your help, but I still couldn't find the problem. I was kinda desperate so I finaly decided to plug in some old 3-com ISA network card I had somewhere. It works fine now. Still, I hate not having it get to work with two normal PCI cards.