[CentOS] Centos6(64) SSH sessions fail irregularly

Folks

I just installed a centos6 system on a 64-bit box.  My methods 
include remote administration, using ssh from a windows machine.  I 
use this method successfully on several Centos5 boxes and one Centos6 
(32bit) machine as well.  However, on this latest one, I get 
inconsistent results with ssh.  This is a server, SELINUX is 
disabled, it's command line only, and I have installed sendmail, 
apache, vsftpd (but haven't used the last two yet).  The machine is 
connnected internally only (behind NAT), so is invisible to the outside world.

In a successful use of SSH just after installing the needed software, 
I installed the DSA keys so no password is used.  Subsequent to that, 
and at seemingly unpredictable frequent attempts, I get results such as

a) A successful SSH login as desired.  Variious commands, sucn as 
"ls" and such, work fine.
b) A successful login, but a write failure with whatever I enter next
c) Connection refused
d) A request for password (followed by failure)

I can't seem to connect these various errors with any activity on the 
server -- I'm monitoring /var/log/messages /var/log/secure and 
/var/log/audit/audit.log, on a terminal directly attached to the server.

The server, and my windows box (which is my admin box) are on the 
same internal IPV4 network (192.168.xxx.xxx), all controlled by a 
Centos5 gateway running just fine which provides DHCP and NAT when needed.

I haven't been able to correlate these failures with anything I do; 
there's no indication on the server that anything is happening.  Of 
course, when the SSH login succeeds, there's a notation in the logs, 
but no evidence of the failures.

I tend to get "best" results after a bootstrap,

With no evidence of anything abnormal showing up in the logs, it's 
not clear where to look.  I would appreciate some clues as to where 
to look next.  The server seems to have functioning internet access 
through the gateway.  I can easily redo the install if needed.  The 
hardware was previously running Win7, so I suspect it's good 
hardware.  It's a roll-you-own machine built with components from 
Fry's, so don't ask me for the model name :-)

I'd appreciate advice or guidance.

David Kurn