1.  If you are running MTS (Multi Threaded Server),then
    try the same connection with (SERVER=DEDICATED) in
    the tnsnames.
2.  your server is NT  then ensure that the port being
    listened on is open as well as setting up a 
    SQL*Net/Net8 Proxy, such as Net8 Connection
    Manager, if none is provided by the Firewall
    vendor. The reason is that, even though the port 
    is open, the TCP layer will REDIRECT the port to 
    another port.
3.  If any of above is not helping you,
    pls enable the client sqlnet and listener tracing
    for further diagnosis.