FTP connection locks me out

I installed CentOS 5.1. With the default installation the only opened
port is 22 and SSH is configured to accept connections. I login to the
system, do a lot of stuff, work with X (using NX), install rpms, etc.
I have even installed XEN without problems. Now, I would like to
transfer some big files (8 GB each) from CentOS to a Windows machine.
Just to make it easy I login to CentOS using ssh and I connect from
CentOS to the Windows machine using FTP. Now I put some files in the
Windows machine. After some time the connection to the CentOS machine
is lost and I can't connect anymore to the CentOS machine. I tried to
restart sshd, network and iptable scripts. No way. I'm locked out. If
I login to the CentOS with the interactive console, I can see sshd is
running and is not blocked (I can do "ssh localhost" without
problems). What can it be? Something related to security or something?

Thank you in advance for your help

Cold
.

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