You'll have to do some diagnostics at the time of network trouble. The four steps I use to troubleshoot network connectivity follow:
1) check your IP address and gateway
2) ping the gateway
3) ping an IP address
4) ping a DNS name
If step 1 shows a bad IP address, your DHCP server isn't giving you a correct IP address probably because of lack of connectivity. If you can't ping the gateway (step 2) you have lost connection. Step 3 should be a public IP address (I usually use my DNS server), and step 4 would be pinging google.com or another big name. If all 4 steps are okay, you have network connectivity, so the problem must be localized to your browser.
It might also help to know your network topology.