This is a very strange problem.

Is there a way to check link status in OS 9? Maybe a link isn't even being made. That may provide a clue. I have an almost-dead 100Mb switch that has a couple of ports that only work with certain computers because of low voltages. You may have an iffy switch in your building or your onboard ethernet may have gone a bit out of range.

Might you consider a wifi card for your PowerBook to run off of your router? That could be cool.