OS 9 to OS X suddenly disconnects

We have four Macs, three on Tiger and one on 9.2. Each one has a static IP and they're all on the same ethernet network.

The OS 9 machine will connect fine to any of the other three and you can browse and move files around on the other Mac. As soon as you try to copy something over from the OS 9 Mac to one of the others, it cuts out after 3 or 4 seconds with this:

iMac via TCP/IP
The file's server connection has unexpectedly closed down.

If the file is small enough to be copied over in a second or two, it will be copies over, but larger files get cut off because the network shuts down after 3 or 4 secs.

It all used to work fine. We've tried changing the ethernet cable, but it didn't help.