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Loss of connectivity through NAT

I've been using Debian (Sarge) as a NAT server for several years and have had 
few or no problems.  Recently, however, I've been experiencing some problems.  
First, I'll lost the connection through NAT to all machines intermittently.  
When I check the server, I can still ping an internet address but I can't 
ping it from any of the machines relying upon the server for NAT.  This 
usually only lasts a short while - say 90 to 120 seconds.  Then the 
connection will start working again and be fine for several hours.

Additionally, on one machine running Gentoo, I lose NAT connectivity 
semi-permanently.  If I check the interface, I can ping the local inteface on 
the server but not the remote interface.  If I attempt to ping, say, Google, 
I get DNS resolution through the same server as is running NAT, but I get 
100% packet loss.  All of the other machines (there are three of them) still 
have connectivity through NAT.  If I take the interface down and bring it 
back up (via ifconfig), I get my connectivity back.  These symptoms make no 
sense to me.  I can't think of any problem on the Gentoo client which would 
allow me to still ping the server and get DNS resolution, but prevent NAT 
from working via the same server.  On the other hand, I don't see what could 
be wrong on the server that would repair or reset itself when I restart the 
interface on the client.  I have checked and rechecked the symptoms, however, 
and that's what I'm getting.  Hell, I'll post a command line session if 
anyone wants to see it.

Any suggestions or clues on what might be going on, or how to go about 
diagnosing the problem would be greatly appreciated.

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