I have had similar experience where, like you, I sometimes get "cannot find server" but more often page freezes and especially drop-down selection boxes on web pages appear to be dead. The only solution I have found is not very elegant but does work for sites you visit regularly. Open NIS, select Options/Web Content, Add Site (specif web site that you want to access, ensure to check mark Scripts = Permit.
So my real problem is when browsing lots of sites that I don't intend to visit regularly. When doing this I usually disable NIS - which I don't like doing.
I would be interested to here about better solutions. I have tried Norton/Symantec themselves, but they were not much help.