I love realtek - the resources they use tend not to conflict with other cards or hardware, they don't use much cpu time, the drivers are mature, and they don't cost much. What could be better? There does seem to be at least one onboard realtek chipset that can have driver issues, but I use the 8169 without problems. But hardware does fail. And any brand of nic can fail in odd ways. I'm guessing you've swapped it out? Bios settings can change if the on-board battery is dead and the system loses power. (It can set to defaults) But bios settings rarely affect nics - you're more likely to see boot problems from a change in drive boot sequence. I don't suppose you have a vpn on your lan? I noticed you use the 192.168.1.x address range, which is one of the most common ranges in the world. If someone connects to your vpn from home or workplace, and if they use the same range, and if theres a bridge, addresses are going to conflict. If you delete your ifcfg-eth0 or ifcfg-eth1 files, centos will recreate them if it sees the nics at boot. But it tends to enable eth0 and disable eth1 or higher. You should have backups of your originals for that reason... I bet you wish you had a tcp/ip based kvm switch system about now...