There may be an path mtu or a link speed negotiation issue. For path mtu to work, icmp unreachable messages can't be blocked by black-hole routers and the echo and echo-reply must be sent end-to-end. So if there is any device that blocks those messages, then the ftp connection is probably using 512-byte payloads instead of 1280 byte payloads.
If the 3020 or the ftp server's interfaces are set to autonegotiate, then try explicitly setting the speed and duplex on those interfaces as well as the switch ports that they connect to. You may have a similar issue on the client end, though you may not be able to do much in the case of remote clients - however you can try to have the remote user do an explicit speed/duplex setting on the client's NIC.
Let me know what you find.
