You must have a mobo chipset with onboard LAN. I had the same problem...my eth0 and eth1 kept uninstalling and the only thing I had registering was my lo loopback. I'm not really sure what the problem was....I went around on the web to look for help and couldn't find it at all. I sent an email to slackware and haven't heard back from them yet...and I went on various nets in IRC and asked but no one seemed to know.
I suggest swinging back to slackware 8.1
I couldn't get it to work right...my box would reboot sporadically, my network would crash...then all of the sudden SAMBA started to crash and my mounted file systems began to unmount on their own...I said, "enough of this shiot" and blanked it out. Reinstalled 8.1 and everything has been going peachy...no problems whatsoever. Even updated a video card and everything still went great. Sorry I can't help you other than sharing in your aggravation and common problem.
I didn't think of it until later...but you should be able to see if there are linux drivers for your mobo chipset. I'd go to the manufacturer's website and check. Might be as simple as that instead of a reinstall. Unforotunately they didn't have drivers for my mobo. :/
<font color=red>To err is human, but to really foul things up requires a computer.</font color=red>
<font color=green>Farmers' Almanac, 1978</font color=green>
TKS