Clients can’t browse Internet through ISA because ISA is waiting for the DNS to answer
Just a quick follow up on the article that I wrote for the ISA Team Blog about ISA stopping answering requests. Last week I was collaborating with Networking Team in another case where ISA was stopping answering because of delays in DNS response. They fixed the DNS issue by changing the registry keys SocketPoolSize and MaxUserPort in all internal DNS Servers using recommendations from KB956188.
Conclusion: keep yourself alert on slow browsing issues and make sure that your DNS is working properly prior to start troubleshooting ISA.