servers disapearing from browser

Hi
We have a recurring problem where some servers will temporarily
disapear from the browser list. It is still possible to put the UNC path in & get to the server but it is not possible to browse to it. It is usually the same servers each time and can be several weeks between each disapearence. They reappear after a while, which could be a an hour or so, to 12 (ish) hours.
Network is all Windows & servers are mainly Win 2003, boxes doing the browsing are all Win Xp. All have latest patches on. There are about 150 servers & 2500 desktops, all on the same domain on a class ‘C’ address range.
Any help & thoughts appreciated.
Thanks