IE6 on win xp sp2 on win2000 server, running through router and offsite to a
3rd party proxy server.

Some of my 200 machines intermittently lose their internet access, giving
the 'page can not be displayed' message, it is not isolated to a certain
room, all machines are set up identically. This 'no internet' access lasts
for anything up to 2 days, then suddenly comes back. They have no viruses and
who ever I am logged in as is the same (including admin).
have reinstalled IE6.
all machines IP, DG and subnet are all correct.
any ideas what i can look at to troubleshoot?