Strange network behaviour

I have got a really strange problem in my LAN. The problem is the LAN connection on the PCs keep going on and off randomly. I have done a lot of things to find out what is going on and how to solve the problem. But so far i haven't solved the problem and I really need some help. I have explained below how my LAN setup is and what I have done so far to solve the problem.

I have 5 Win XP pro sp3 PCs in a LAN. 1 PC is acting as a server and the the rest are clients connected to that server. The LAN is part of a bigger MPLS network connected through a router. Here are som details about the setup:

* Every client is placed in its own room and connected to a switch through a wiring closet.
* The server is connected directly to the switch
* The switch is connected to the router
* All Pcs are using IP
* All PCs use the router as the standard gateway.
* All PCs have static IPs.

I have done a lot of different PING requests to different PCs and the router. The PING request goes fine and than suddenly returns "hardware error" and after that "request timed out". Its totallly random when and how this happens. When this happens, a message comes up saying a LAN cable has been disconnected.

I have tried to change the netcard and the cables. I have tried to connect the PCs only to the switch with no link to the router. At this point the PCs were set to have dynamic IPs. I have tried to have PCs only connected to the routers LAN ports with no link to the switch. None of these methods help at all. The Router is from our ISP and they have done some diagnostics on it and concluded there is nothing wrong with the router.

I have tried to connect a PC directly to the router instead of through a wiring closet.

The router can ping the rest of the MPLS fine without any problems.

Nothing has helped so far and I am really stuck at this point. Any help will really be appreciated.

Thanks in advance. I look forward to your comments.

- Mudasar
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  1. I have now minimized my setup to make it easier to find the problem. I have now only 1 pc connected. Both the router and the switch has been changed to new ones. I have changed the netcard in the PC and the cable is changed as well. The problem is still there though.

    Any suggestions? I am thinking it must be some kind of bad setting on the PC? I dont know where to look though. I have thought of using a network diagnostic software, but i dont know any that can help in my case.

    I have also been thinking if this problem can be triggered by a virus or something? Is that possible? I have scanned for virus with F-secure and the PC looks clean.

    Any ideas or suggestion will appreciated as I am stuck here.

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