Hmm. Try reinstalling the OS and trying again, like you said you were going to do with Tiger, but completely reformat the HDD via Disk Utility while booted from the installer and do a completely fresh install. If it continues to happen, has to be a problem with the board. I have connected my Clamshells via Ethernet (one is even running 8.6!) and no problems.

Also, I would consider Panther (10.3) for the G3s over Tiger (10.4). It can run, but it isn't going to run fast.