Network drives disappears frequently

We have a 2003 server with shares. We use the "Connect [drive_letter] to
\\server\users\[user_id]" in the user's profile (in Active Directory Users
and Computers) to make their folders available.

Once or twice a week one user has the "Z:" drive disappear. All others are
there. She can log out or reboot and it will not come back. If I use the
"Connect [drive_letter] to \\server\users\[user_id]" in the user's profile to
change it to something else then change it back, she gets the drive back.
Sometimes it'll come back after a few days (like over the weekend).

Any ideas?

TIA

jwkh