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Some PC's on a Windows server 2003 domain keep dropping their internet connection

For the last 3 weeks, about 10 workstations on this main subnet (90 connections) of a Windows 2003 single  domain  randomly drop their Internet connection sometimes a few times a day.  Consistently, running  'ipconfig /flushdns and ipconfig /registerdns fixes the issue, but only temporarily.  Then the problem comes back.  When the machine has this problem, internet addresses can be pinged, but not browsed to, even if the address is put into the browser.  The age of the machines does not seem to matter.  Simply rebooting sometimes fixes the problem, but sometimes does not.  DCdiag DNS tests on the 2 DCs are clean.  Access to internal resources is OK, except, randomly, to Live communicator.  I also noticed that a couple of people are intermittently unable to access Live Communicator, even when they CAN get to the Internet.   Most people are completely unaffected. This is a new problem, and happens to the same machines.   I have taken machines off the domain and re-added them, to see if that fixes the issue permanently. I will know if that worked tomorrow.  There is no common thread in the local event logs. The server logs are clean of events that would affect this.  I am not sure where to look next,  any assistance would be appreciated.  I can post whatever you think would help.
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quaybj
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quaybj
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petelettinCommented:
Do you use a proxy server?

When a FQDN is pinged the address is resolved by the local machine.
If you use a proxy the address is resolved by the proxy server.

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quaybjAuthor Commented:
Thanks petelettin,
We are not using a proxy server.
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quaybjAuthor Commented:
correction to the description: Internet addresses CANNOT be pinged consistently when this problem occurs.
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pmannoCommented:
Have you looked at the output of ipconfig /all and route print on the client machine during the connection drop?  Does the ipconfig change?  Does the client lose the default gateway?  Can clients ping and resolve local network devices?  What OS are the clients running?  I've found that some Vista machines randomly drop the routing table entry for the default gateway.
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quaybjAuthor Commented:
Thanks pmammo.
I rebooted all the switches and the problem went away.  I am not convinced the real solution has been found.   I will take your suggestions to investigate routing under advisement for next time (and I'll bet there will be a next time).  Usually when the switches need to be flushed, no one can connect to the Internet, so I was looking for a different problem since the issue was not widespread.  
We are not running any Vista clients, only XP.  Clients could continue to ping and resolve local network devices;  ipconifg /all showed they were pinging the gateway.  They could not use Live Communicator, though Exchange kept working.  A mixed bag of symptoms.
I am awarding you some of the points because your suggestions make sense.
Thank you for your comments.
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