It turns out the 2nd ethernet card keeps changing its device name every time the PC reboots. Instead of being eth1 it keeps changing to dev followed by a number, like dev8761.
The two cards are Intel Pro 100S and I updated to the latest drivers. I also went into BIOS and tried with/without plug/play and also manually set the IRQ for the PCI slots. But it still keeps changing.
kernel version 2.4.21-27.0.4.EL
I found another thread here with same problem but no solution