As long as this doesn't happen all the time, I personally wouldn't worry about it. If you want to investigate it further, then it can be caused by a large number of things - network issues, proxy problems, DNS issues, for example.
thanks joe ,
but this error happens every day . we have a proxy too in our environment. So what could be the reason .?
It's probably related to the proxy server. Anything interesting in the logs?
on 11/16/12 1:18:11 AM CST
Does it happen at the same time every day? If so the proxy may be configured to only allow access at certain times...