File Server Permissions

Our network has a file server running 2000 server with a “user” share, the end users are about 50/50 2K and XP with Office Xp,2003 and 2007 (saving 2003 compatible). My problem is that random files when accessed will show that user does not have permission, but if you add a generic domain user, click apply and all specified permissions come back. One PC has the problem routinly in that if a user, admin or other, creates a file and saves it, no one can access it without going and adding permissions. Even from the user/machine that owns it. Does anyone have any ideas?