Listing the symptoms observed.
1. The G4 PowerMac's transfers stall, notably when sending files out.
2. If the G4 PowerMac is connected to a 10baseT hub, which is then connected to our router, transfers work fine. Slower, but fine.
3. Transfers with both my laptop and the G4 PowerMac connected to said 10baseT hub work fine.
4. Another network card, plugged into an open PCI slot of the G4 PowerMac has the exact same problems. Not a hardware NIC problem
5. Restarting into 10.3.9 on a backup drive does not fix issues. Hence; not a software problem.
6. Transfers from my laptop to all other machines in the house (two mac minis) all work fine.
! It just occurred to me that the PC box's ethernet port might actually be functional. It's just not auto-negotiating correctly with our router. Huh. Interesting possibility, that.
Current fix: Disable automatic ethernet speeds, set to manual, 10baseT, Full-Duplex.
Possible permanent fix: Replace router?