I have problems with eth1, sometimes it works just fine and on the next boot it won't work. I've tried to find the solution by myself without success, so now I'm hoping somebody here would have an idea what to do.
Eth1 is an integrated NIC (Info Center says it's Intel EtherExpress PRO/100 VM), and ifconfig shows the correct settings for it. It won't actually send or receive anything however. I checked by ping and by ethereal.
Then I went googling for some info. And tried again. Ethtool shows a lot of "no", "not reported" or "unknown" settings and running the online and offline tests gives FAIL.
At the moment i'm on a hd-installed knoppix, but the same problem persists if I use the live cd. (Knoppix 3.6)
On boot both eth0 and eth1 are found, but the line that mentions Intel PRO/100 shows only during the times when eth1 decides to work.

I tried booting with the 2.6 kernel and this time eth1 didn't come up at all. It just reported SIOCSIFFLAGS: Resource temporarily unavailable (or something). It gave the same error when trying ifup.
I googled that error and found out that it was usually caused by IRQ conflicts. I went to BIOS settings and tried disabling everything extra that might be causing IRQ conflicts (USB, parallel and serial ports, onboard sound) , tried disabling PnP and even tried switching the correct IRQ slots to legacy ISA. Well, according to /proc/interrupts and /proc/pci (and lspci -v) there was nothing using the same IRQ after that, but still it gave the same SIOCSIFFLAGS error.

Of course it's possible that the problems are caused by hardware, but I'm too much of a newbie with Linux to make that call by myself. And since the PC doesn't have any more free PCI slots (a small box), it would be quite important to get the onboard NIC working.

So, if anyone has any good ideas, i'd be glad to hear them. And thanks in advance.