Port forwarding to a linux box with a D-Link 302G problem

Hi

Any ideas on the following:

I have proftpd, ssh and Apache running on a linux server on my local
network. These services are working fine and I can connect and use them from
a windows box on the local network.

I have a DLink 302G ADSL modem. I have tried to setup port forwarding for
the FTP, SSH and web services. I can forward http requests from external
port 5080 to port 80 on the linux box in the internal network, and that
works fine. (using http://61.XX.XXX.XX:5080) When I try to forward ports
20,21 and 22 the same way I have problems. I know the services are up and
running because I can login from the internal network. When I try to ftp
from the internet I get the login prompt but it isn't the server, it is the
modem. When I ssh I get a straight out connection failure. I've also tried
to use higher ports like 5021 and 5022 and forward them to 21 and 22
internally. Same thing. I know that the ports have opened on the firewall
because port scanning the external IP shows this to be true. Also, if I
forward port 23 and try the same with telnet I get the modem's telnet login
not my linux server. Has anyone seen this happen before or have any ideas?

Thanks in advance.

SJ