I have had some similar circumstances in the past, only for me it was the other way around. That is, YouTube would tank for me while all else was lightning. Still, I would look into and resolution as a possible cause. What do you have your clients set to use for DNS? Is it an internal DNS server? If so, is that server set up to forward to an external DNS that your ISP has provided you? When you are testing outside of your firewall, what are you using there for DNS? Many times I have seen things set up to where the clients primary DNS server is set to an internal server and their secondary is set to an external server, and it just "don't work that way". They should all point to one or two internal servers only. And then those servers should have your ISP's DNS set up as forwarders. Also if you have no forwarders configured, you are asking your internal servers to query the root internet servers for common DNS queries. They will eventually get you an answer ... but it's very slow.