Intermittent network problem

My customer has a small LAN
All machines are "the same" (same original image) and run Windows 7
They are connected to a switch, itself connected to a router
The router is connecte to an ADSL box to the ISP
Some of the machines (a couple of them) have intermittent network problem, accessing the Internet.
We tried to figure out why but failed.
We verify the customers employees saying by running an endless ping from their machines to a site on the Internet and indeed sometime it fails (timeout) for around 30 seconds then again okay
For info, pinging from the router to the internet is always OK
We are looking for suggestions of what to test/check
For info, there is no firewall on users machines
The entire network is wired.
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When it the Internet pings fail can the same computer still ping the router? What you need to do is segment the network and isolate the point at which it is failing. It could really be anything at this point. I would have three pings running simultaneously. Have one ping the Internet and another ping the local router and yet another ping another host on the network. That lets you isolate it with one failure. If all pings die then it's the switch, cable from your computer to the switch, or NIC. If the Internet and router pings die the. It's your router or cable from the router to the switch. If just the Internet dies then it's your isp, modem, or the cable from the modem to the router.
Sounds like hardware.  Could be a NIC card going bad, or a loose cable connection.

If you run the endless PING on several machines at once, do they all get timeouts at the same time?  Do some of them keep going?  Try endless PING within the LAN as well as to an external IP.
I would try plugging the pc directly into the adsl  box and run the same test..
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LeTayAuthor Commented:
I contacted my colleague who is "on site" for this customer
The situation is a little bit different from what he told me originally
I will come back with more information in the coming hours
LeTayAuthor Commented:
We finally located the guilty : the DNS server
Indeed, changing PC DNS (not using that server) and we had no Internet problem on the machine
Also, previous test shown that when a PC had intermittent problem to "solve" DNS name, a ping from the DNS server to an external (Internet) site failed.

Now the question is the following : are there tools (or ideas) to find why the DNS server sometime (but often, and each time for around 30 seconds) does not succeed resolving names ?
Try taking a look at DNS forwarding.

2008 Dns Fowarding

2003 DNS forwarding
LeTayAuthor Commented:
We will have a look at that soon but in a first reading, it looks okay
LeTayAuthor Commented:
Nothing special in DNS log files
Is it possible to investigate in depth ?
For example is there a way to tell the DNS to trace more info in the logs ?
Been following this discussion with interest, but I repeat my original suggestion.  Still sounds like hardware.  Could be a NIC card going bad, or a loose cable connection.

Try an endless PING from the DNS server machine to the router and see if there are any lost packets.
LeTayAuthor Commented:
Sorry to come so lately with an answer
In fact, it is one of my colleague who is in charge with this network
He had other priorities
I come back with more info as soon as I get them
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