Please make sure you can ping/telnet to your proxy server (if you are using any). Telnet to proxy IP on port 80 or 8080 whatever is being used. Moreover, if there is no proxy server in place, you might need to look into the NAT table of your firewall to make sure it has not exceeded the limits (very unlikely though).
By the way, does the edge firewall shows any particular log while you see the internet down?
More thoughts coming up soon
In my network for the several Users internet connectivity goes down for a moment but they can able to ping to the Internet Destination IP. What wouild be the reseaon for it?
As per the behavior mentioned it is due to DNS issue at the time of problem DNS is unable to resolve the ip address of the web site but at the same time they are able to ping the public ip's.
How users used to access internet in your network via proxy or via natting ??
Hope to Help !!
It is happening only with natted ips not for the proxy through accessing Users.
Users are accessing via natting is DNS is configured in router where you have configured natting,if yes what is the dns server ip because at the time of problem you need to check that dns server is up and reachble or not.
Hope to help !!