This is not a browser issue (and least I assume it isn't) since I get the same issues with IE7 or Firefox.
What is happening is that I get a page cannot be displayed error message the first time an attempt is made to access a webpage - it usually (but not always) connects on the second or third attempt.
PCs are all running Windows XP and are part of domain where the server is running SBS 2003. PCs point to the server as the DNS server.
Physical connections are modem - firewall - switch - server and pcs.
Same problem on the server.