Worstations Intermittently Lose Connection To Network

On my office network, the domain controller is a Win2000 Server. Ever since I began upgrading workstations to Windows XP two or three years ago, I have had difficulties. The machines running XP Pro lose connection to file shares on the server (and other fileservers) after a period of inactivity, sometimes as short as five minutes, sometimes as long at 2 hours. When the user tries to open a network share, Explorer hangs.sometimes after 3-5 min a username/password dialog will ask you to authenticate using a different user account than the one you're logged in under, then your connection is restored for the time being.other times, you end up getting a "file not found" or "you do not have permission to access this network resource" error. I've been dealing with this for two years, and recently it's gotten a lot worse on one computer, so I need to fix it badly.