I wish I knew. Just tried again, but all seems fine with one to four small files.
-check mail to validate internet access in both clients
-wait for "stable" connections in LW Connections pane (>8 mins, besides self) in both clients (each shared folder only has 5 files)
-new search for "canon" [Pachelbel--it's not a popular mp3] on the G4 Mac or v.v
-download the 4 ethernet listed files
-check connections pane on the G4; Monitor pane on the G3
-check mail to confirm internet access.

btw, the speeds are great (I read the tooltip reports as estimates). The G3 reported the four 4.5MB files uploaded @236-467 KB/s; the G4 said 192-316 KB/s
The Process Viewer on the G3 did occasionally show some unusual activity--??? user with a process name in oriental-like characters. Console Log hasn't entered anything since the first crash, and the loss of connections could just be the usual.

I'll try again with some new, unhashed files later. The disable untrapeer settings are different in both, too

sorry trap_jaw--I missed your post. Yeah, and my kids aren't going to like that I can search their shared files!