We are experiencing weird issues whose major symptom is slow, or no, loading of new websites. When visiting a page you haven't visited, it will either take a very long time, or not load at all. Refreshing once or twice will usually bring the page up. From that point on, clicking new links on that page will be as fast as normal. This occurs on all systems, with all browsers. This is a new issue that started on Monday. There were no configuration changes over the weekend. We have two ISPs, and the problems occur on both ISPs, so it's not an ISP issue.
It seems like a DNS problem, but DNS resolves very quickly. Looking in the DNS logs when turned on show very quick responses and no errors. Changing internal and external DNS servers made no difference. Visiting web pages by IP address made no difference. Pinging internal and external by name always returns immediately.
While it mostly presents as an internet issue, we do see occasional issues internally. Some dropped pings (by name and IP), some issues accessing local services (NAS device, internal web interfaces). However, these are very very sporadic.
We have a Sonicwall and have played with turning things off one at a time, and everything off at the same time (App Control, content filtering, DPISSL, antivirus, antispam...), as well as administrator logins. This made no difference.
We restarted all our equipment, and nothing seems to have an impact. The Sonicwall is not stressed, the Cisco switches are not stressed.
Any ideas? We could wireshark, but our network has 3000 devices on it, so I don't think fishing through Wireshark will be very fruitful.