iTunes - Losing NAS Library

I have my iTunes library stored on a NAS attached to my LAN. This works fine, *except* every time I reboot my Mac, when iTunes reverts to the default local drive library location, and I have to reset the location in Preferences, then wait for iTunes to rescan. This happens even though I have the NAS volume auto-mounting at login (as an entry in Accounts/Login Items) and have the necessary password information stored in the keychain. Even if I manually browse to the volume in Finder before running iTunes the first time, it switches back to the local library location. Is there a way I can get this setting to stick, so I don't have to do this every time?