NIS or NFS problem

Hi,
    We are using a freeBSD server which acts as NIS,NFS and DNS serer.The server configuration is as follows :P4 1.7Ghz ,3*128MB . We are using 4 nos of DAX 5016 switches with 16 ports .There are about 50 clients.All client machines have linux installed in them .Our network is using cat5 cables and has 100Mbps bandwidth.the server is having about 80 users accounts and their homes in it.The home is mounted using NFS.
     The problem is:
       During shift change when there is login from most of the machines ,the network hangs.The problem mainly occurs when users login to NIS server at shift
change.The network is slow at that time.It takes 10 minutes to login.When we
type a command (eg.#ls), result of the command takes 1 min to
display.There is delay when we move from one tab to another tab of any
browser(mozilla or opera).But the problem stops in about 30 to 45 min.(when every one has logged in).During this time the server load is 0.05,0.03,0.00
 .
        When I check the logs there are 2 messages
      ypserve[115]:res_mkquery failed
      ypserve[115]:DNS query failed

In the client[LINUX machines] the error is
 RPC:sendmsg returned error 22
 lockd:RPC call returned error 22

We have recrimped the network replaced all switches ,but the problem has not been solved.I think it is the problem i\of NIS or NFS. what could cause this  problem and how could I fix it?

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Santosh