FC6 ssh times out to one particular host

I have just migrated from using a Solaris box as my work system to
having Fedora Core 6 on my desktop PC.

I have a number of ssh 'pinholes' in the company firewall to let
me connect to external systems.  These are to four external systems
and from the Solaris system they all work still. From the new FC6
system they all work *except* for the one to my home system.  Thus
the basics of ssh security etc. would seem to be OK.

The one that doesn't work is as follows:-
    ssh client - Fedora core 6, OpenSSH_4.3p2
    sshd host - Slackware 11, OpenSSH_4.4p1


With -vvv on the client I see:-

    OpenSSH_4.3p2, OpenSSL 0.9.8b 04 May 2006
    debug1: Reading configuration data /etc/ssh/ssh_config
    debug1: Applying options for *
    debug2: ssh_connect: needpriv 0
    debug1: Connecting to home.isbd.net [84.45.228.40] port 22.
    debug1: Connection established.
    debug1: identity file /home/chris/.ssh/identity type 0
    debug3: Not a RSA1 key file /home/chris/.ssh/id_rsa.
    debug2: key_type_from_name: unknown key type '-----BEGIN'
    debug3: key_read: missing keytype
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug2: key_type_from_name: unknown key type '-----END'
    debug3: key_read: missing keytype
    debug1: identity file /home/chris/.ssh/id_rsa type 1
    debug3: Not a RSA1 key file /home/chris/.ssh/id_dsa.
    debug2: key_type_from_name: unknown key type '-----BEGIN'
    debug3: key_read: missing keytype
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug3: key_read: missing whitespace
    debug2: key_type_from_name: unknown key type '-----END'
    debug3: key_read: missing keytype
    debug1: identity file /home/chris/.ssh/id_dsa type 2
    debug1: Remote protocol version 1.99, remote software version
    OpenSSH_4.4
    debug1: match: OpenSSH_4.4 pat OpenSSH*
    debug1: Enabling compatibility mode for protocol 2.0
    debug1: Local version string SSH-2.0-OpenSSH_4.3
    debug2: fd 3 setting O_NONBLOCK
    debug1: SSH2_MSG_KEXINIT sent

and then after a long wait it times out with:-

    read from socket failed: Connection reset by peer


A Google search has turned up one suggestion that it might be to do
with MTU and fragmentation but I have tried setting the MTU to 576 at
both ends and it still acts exactly the same.

Other Linux boxes at the client end do work (one older Fedora, I think
and one kubuntu) although another Fedora Core 6 installation fails in
exactly the same way for this one host.

Can anyone suggest any reason for this problem and how to fix it? I'm
stumped.

-- 
Chris Green (chris@halon.org.uk)