browser doesn't find/remember servername each time

i have build an internal website for my company (an intranet)

it uses integrated windows nt-accessrights for determing if a user has access to a page or not. It worked great for 3-4 months but lately i discovered something weird:

when i enter the name of the server in the browser he can't find it anymore. When i enter the IP adres it works again. but if i keep trying with the servername sometimes (after trying e.g. 15 times) it suddenly works

Then there are also differences by workstations using WinXP and workstations using Win98se. WInxp stations always work with the name, but when i enter the ip-address it asks for username and password. Win98 stations only work with the ip-address and don't ask anything...

I really don't get it...

one thing which may have something to do with it is that about a month ago we installed a new Isa-server (proxy) on which my site runs. i don't know if this is the cause of the problem cause i only noticed it this week (so it's running fine for 3 weeks).

i hope i described this a bit understanding for u readers....if anybody has some kind of clue...plz scream it out

Harold