I've got a RB493G running v5.18 that configured with ether1 as the WAN port and ether9 as the LAN port. Basic firewall rules and NAT with a FTP server on the LAN.

If I start a FTP transfer, it will begin then anywhere from 3 to 5 minutes later fail. Logs on the router indicate that "ether1-WAN link down" and about 4 seconds later will come back up. If I limit the transfer speed from the FTP server(speed limit set on the FTP itself, not RB) to 256 Kb/s or slower, transfers complete and ether1-WAN stays up. Anything over that threshold and the issue pops back up again.

I've seen some posts on port flapping, could this be related? What can I look for to help me troubleshoot?

Update: I can also reproduce the "ether1-WAN link down" behavior by running a bandwidth test to another MT router(RB450G running v5.4). The test indicates I'm getting the full 5MB/s from the ISP, but the link will go down after 3-5 minutes of running the BT.