Samba failing to answer request shortly after startup

[Service Informatique CHSR]

Hello,
I've been digging a lot before I send this request, but I don't seem to find
what I need.
I'm running Linux RedHat 5.0 with all current patches, and Samba 1.9.17p4.
Here is the problem:

As soon as the samba service starts, one can connect from any client, and it
works perfectly (browsing, conecting, copying files back and forth, ...).
But after ~10sec, samba refuses all new connections, as if it has died.
Let me explain that: the connection that have been already made continues to
work as expected, but the server disappear from the browsing list, a ping to
server with its name states "bad IP adress", when I try to open '\\server'
from Windows 95, if says "'\\server' is not accessible: computer or share
name couldn't be found". (it is exactly the same message as I get when I
type '\\InexistantServer' . just like as if the server does not exist any
more.

>From the linux box, nmblookup and smbclient -L doesn't show the server nor
its share, as if it does not exist. smbclient '\\server\share' from the
linux box works but after a llllllong delay.

Again, the connection that were made shortly after the service startup
continues to work perfectly! (netstat from linux or the client shows a
'nbsession' from the client to the server).

I suspected a configuration problem: even after a re-install from scratch,
the same problem arise. I tried to change the hardware (new PC, new mother
board, new network card, new everything), and the problem was still there.

Note that all other services work correctly (e-mail, httpd, telnet, ftp,
...)

Then I had an idea: I tried to put the linux box and some client *alone* on
a HUB, disconnecting them from our network. And it works OK !!!!!

So, I guess that *something* on our network that causes Samba to fail
shortly after startup, keeping connection already established, refusing all
new connection attempts, shutting down browsing, etc...

The question now is: *what*????

Perhaps someone can give me a clue, a guideline, something to check or
whatever that could help me. The odd thing is that *only* Samba fails to
operate that way: all other services work as expected.

All suggestions are greatly appreciated. I hope my long story did not make
you sleep :-)

Jephte CLAIN