*** Configuration: ***
I have a Windows 2003 Web Server edition, running IIS 6 and Argosoft
Mail Server Pro. Let us call it SERVER2. No other software was ever
installed. This box is one of the 2 boxes connected behind SonicWall
TZ170 firewall. The other box has Windows 2000 server and never has
any problems, stays online for months, let us call it SERVER1. Both
servers have internal (private) IP addresses ? 10.0.0.1 and 10.0.0.2,
firewall has one-to-one NAT.

*** Problem: ***
Every few days SERVER2 loses network connection. It cannot be reached
(or reach out) neither by ping nor any other protocol, like HTTP,
SMTP, POP3, PPPT, Tem Services etc from the inside of the LAN (from
another server before the firewall) as well as from WAN (through the
firewall). The server itself remains operational, no errors, no
messages, does not slow down, just no connection. Connection gets
restored if I unplug the Ethernet cable and plug it back again or if I
restart the server. Restarting the firewall does not make any
difference, it does not restore connection.

*** Other Facts:***
1.	I tried to connect SERVER2 to different physical ports of the
firewall, still the same.
2.	I had both servers connected to a switch, so even if the firewall
was bad, if I could log in SERVER1 through Terminal services, even if
firewall caused me connection problem to SERVER2, if the SERVER2 was
OK, I could still ping it, but it shows no connection.
3.	Event log shows no errors whatsoever around the time when the
server2 loses connection.
4.	Server2 has only one network card, which was replaced, but the
problem remains the same.
5.	NIC has only one static IP address (10.0.0.2)
6.	Local Area Connection properties show only 3 items ? ?Client for
Microsoft Networks?, ?File and Printer sharing for Microsoft Networks?
 and ?Internet Protocol (TCP/IP)?.
7.	I need file sharing because I do a backup from one server to
another using NTBackup. Connection is getting lost not close to the
time of the backup start or finish and it is not happening during the
backup.
8.	I do not see any pattern of times when connection is getting lost. 
9.	Firewall does not show any attacks before the connection gets lost.
10.	Server is not frozen when connection gets lost, it responds to the
keyboard and everything works fine except that there is no connection
11.	When connection is lost, the lights on the network card are still
on and blinking.
12.	The same version of Argosoft Mail Server Pro is running on 2 more
servers that never have any problems with anything at all.
13.	When connection gets lost, the event log does not show that,
although if I unplug Ethernet cable, event log shows that the
connection is lost, then when I plug it back, it shows that connection
is restored and the server works after that for the next few days.
14.	Server1 always remain fully operational when Server2 has no
connection. Also the firewall is fully controllable from bot WAN and
LAN.

I hate to ask this question but......have you tried replacing the
network cable? This really sounds alot like a cabling problem but I
would assume you already tried that if you replaced the NIC.

I was told by my colocation provider that they replaced the cable when
they replaced network card a while ago. Nevertheles thank you very
much for your response, much appreciated.

Is the NIC on the HCL list for W2k3?  Also have you tried changing the
NIC's properties Media type to Full Duplex 10MB/100MB otherwords
disabling the auto selection to see if there is an issue between the
switch and the NIC.  Just an idea...

If have pretty much this exact same problem: 
although I have the webserver on SERVER1 (which uses MS ICS to share
my bradband connection) and have no server2..... every few says i am
usable to access the net from ANY of the PCs connected to SERVER1 and
I am unable to access the webserver running on SERVER1 from any
external computer (over the net as oppose to LAN).  Someone did
suggest that I using win2k3 as a dchp server and using ICS on it at
the same time might cause a conflict but i am away from the computer
for a week and so cant test this theory atm.
I just wanted to know if you had managed to resolve this issue?  When
the internet is down from the client machines I am still able to
access shared files and on SERVER1 and other clients and can PING all
computer including SERVER1.

THANK YOU SO MUCH TO EVERYONE WHO SPENT TIME TRYING TO HELP.
UNFORTUNATELY I DO NOT HAVE ANY SOLUTION TO REPORT TO YOU. I JUST
WANTED TO CLOSE THE CASE AS I BOUGHT A NEW SERVER.

THANK YOU ALL AGAIN, I MUCH APPRECIATE YOUR CONTRIBUTION!