I currently have ISA Server 2000 SP1 running on a Windows 2003 Enterprise Server box at the moment. What we seem to be having problem is with sites like Yahoo Mail or Expediacorporate.com where users log in, but on occasion, like for Expediacorporate.com, they plan out something and submit it, they get a 'Cannot Display Page' with 'Cannont find server or DNS Error'.
One user states that if they wait a few minutes, try it again, it works fine.
I look into the proxy logs and I don't see any problems, I do see the result 12209 for anonymous attempts (I currently have it set to check for authenication for logging purposes.) but obviously, the problem isn't the 12209 denial, but a simple, "Cannot Find Server or DNS Error".