Peculiar behavior of ssh: hangups, changing host key

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I installed Centos 5.5 on a new computer (SuperMicro H8DGU) yesterday. Some odd things happen sporadically when I connect to it by ssh from a terminal emulator. Mostly I use Van Dyke's SecureCRT on a Windows machine, but not exclusively.

1. Occasionally the connection drops, and when I log in again I'm told "The host key sent by the server is different from the host key stored in the host key database." Then it gives me the MD5 hash of the host key fingerprint. The odd thing is that this hash alternates between two different values! Just two!

I just ran ssh-keygen -l -f /etc/ssh/ssh_host_dsa_key.pub, and I recognize the output as one of the two hash values. The date on the file is yesterday afternoon, so that hasn't changed.

2. Sometimes there is an error message "Auth User/Pass with PS...fail...Please reconnect!." This is preceded by an "unspecified GSS error", if I remember correctly.

I'm using password authentication.

I can't find any relevant error messages in /var/log/secure, just "password accepted" and "end session" lines.

I have compared this machine to a similar machine (Scientific Linux 5.5) that works properly. The sshd_config files are identical. So are /etc/pam.d/sshd and /etc/pam.d/system_auth. In fact I haven't messed with anything in the sshd configuration.